Mark OffscreenPageLimitProperty as obsolete and remove unused code (#31306)

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## Description

This PR marks the `OffscreenPageLimitProperty` and all related methods
as obsolete throughout the .NET MAUI codebase to indicate that these
APIs are deprecated and should not be used in future development.
Additionally, it removes unused code that was identified during the
review process.

## Changes

### API Deprecation
Applied `[System.Obsolete]` attributes to the following APIs in
`Microsoft.Maui.Controls.PlatformConfiguration.AndroidSpecific.TabbedPage`:

- `OffscreenPageLimitProperty` - The BindableProperty definition
- `GetOffscreenPageLimit(BindableObject)` - Static getter method
- `SetOffscreenPageLimit(BindableObject, int)` - Static setter method  
- `OffscreenPageLimit(IPlatformElementConfiguration<Android,
FormsElement>)` - Extension method
- `SetOffscreenPageLimit(IPlatformElementConfiguration<Android,
FormsElement>, int)` - Extension setter method

### Implementation Updates
- Added `#pragma warning disable CS0618` around existing usage sites to
suppress obsolete warnings in internal implementation code:
- `TabbedPageRenderer.UpdateOffscreenPageLimit()` (Compatibility layer)
- **Removed unused code**: Deleted the `UpdateOffscreenPageLimit()`
method from `TabbedPageManager.cs` since no code was calling it
- Preserved the `UpdateOffscreenPageLimit()` method in
`TabbedPageRenderer.cs` (compatibility layer) where it is actually used

### Code Cleanup
The unused `UpdateOffscreenPageLimit()` method in the modern
`TabbedPageManager` implementation was removed as it was never called,
while preserving the same method in the compatibility layer where it's
properly utilized.

## Impact

- **Backward Compatibility**: All existing code continues to work
unchanged
- **Developer Guidance**: Developers using these APIs will now receive
compiler warnings indicating deprecation
- **Internal Implementation**: Framework internals continue to function
with appropriate warning suppression
- **Code Quality**: Removed dead code that was not being used
- **Future Direction**: Signals to developers that these APIs may be
removed in future versions

## Testing

- All existing TabbedPage unit tests continue to pass
- Core.Controls and Compatibility projects build successfully
- No breaking changes to public API surface beyond the addition of
obsolete attributes

This change follows the established pattern for deprecating APIs in the
.NET MAUI codebase and provides clear guidance to developers about the
future direction of these APIs.

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Author: jfversluis

src/Compatibility/Core/src/Android/AppCompat/TabbedPageRenderer.cs

@@ -573,7 +573,9 @@ void UpdateIgnoreContainerAreas()
 
 		void UpdateOffscreenPageLimit()
 		{
+#pragma warning disable CS0618 // Type or member is obsolete
 			_viewPager.OffscreenPageLimit = Element.OnThisPlatform().OffscreenPageLimit();
+#pragma warning restore CS0618 // Type or member is obsolete
 		}
 
 		void UpdateSwipePaging()

src/Controls/src/Core/Platform/Android/TabbedPageManager.cs

@@ -477,11 +477,6 @@ void UpdateIgnoreContainerAreas()
 			child.IgnoresContainerArea = child is NavigationPage;
 	}
 
-	void UpdateOffscreenPageLimit()
-	{
-		_viewPager.OffscreenPageLimit = Element.OnThisPlatform().OffscreenPageLimit();
-	}
-
 	internal void UpdateSwipePaging()
 	{
 		_viewPager.UserInputEnabled = Element.OnThisPlatform().IsSwipePagingEnabled();

src/Controls/src/Core/PlatformConfiguration/AndroidSpecific/TabbedPage.cs

@@ -108,19 +108,22 @@ public static IPlatformElementConfiguration<Android, FormsElement> DisableSmooth
 		}
 
 		/// <summary>Bindable property for <see cref="OffscreenPageLimit"/>.</summary>
+		[System.Obsolete("OffscreenPageLimitProperty is obsolete. This property will be removed in a future version.")]
 		public static readonly BindableProperty OffscreenPageLimitProperty =
 			BindableProperty.Create("OffscreenPageLimit", typeof(int),
 			typeof(TabbedPage), 3, validateValue: (binding, value) => (int)value >= 0);
 
 		/// <summary>Returns the number of offscreen pages are cached in memory.</summary>
 		/// <param name="element">The platform specific element on which to perform the operation.</param>
 		/// <returns>The number of offscreen pages are cached in memory.</returns>
+		[System.Obsolete("GetOffscreenPageLimit is obsolete. This method will be removed in a future version.")]
 		public static int GetOffscreenPageLimit(BindableObject element)
 		{
 			return (int)element.GetValue(OffscreenPageLimitProperty);
 		}
 
 		/// <include file="../../../../docs/Microsoft.Maui.Controls.PlatformConfiguration.AndroidSpecific/TabbedPage.xml" path="//Member[@MemberName='SetOffscreenPageLimit'][1]/Docs/*" />
+		[System.Obsolete("SetOffscreenPageLimit is obsolete. This method will be removed in a future version.")]
 		public static void SetOffscreenPageLimit(BindableObject element, int value)
 		{
 			element.SetValue(OffscreenPageLimitProperty, value);
@@ -129,12 +132,14 @@ public static void SetOffscreenPageLimit(BindableObject element, int value)
 		/// <summary>Returns the number of offscreen pages are cached in memory.</summary>
 		/// <param name="config">The platform specific configuration that contains the element on which to perform the operation.</param>
 		/// <returns>The number of offscreen pages are cached in memory.</returns>
+		[System.Obsolete("OffscreenPageLimit is obsolete. This method will be removed in a future version.")]
 		public static int OffscreenPageLimit(this IPlatformElementConfiguration<Android, FormsElement> config)
 		{
 			return GetOffscreenPageLimit(config.Element);
 		}
 
 		/// <include file="../../../../docs/Microsoft.Maui.Controls.PlatformConfiguration.AndroidSpecific/TabbedPage.xml" path="//Member[@MemberName='SetOffscreenPageLimit'][2]/Docs/*" />
+		[System.Obsolete("SetOffscreenPageLimit is obsolete. This method will be removed in a future version.")]
 		public static IPlatformElementConfiguration<Android, FormsElement> SetOffscreenPageLimit(this IPlatformElementConfiguration<Android, FormsElement> config, int value)
 		{
 			SetOffscreenPageLimit(config.Element, value);