fix: Remove virtual GetEventChannel from FeatureProvider (#401)

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## This PR
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This pull request includes changes to the `EventChannel` handling in the
`OpenFeature` library to improve code consistency and reliability.

Improvements to `EventChannel` handling:

*
[`src/OpenFeature/EventExecutor.cs`](diffhunk://#diff-f563baadcf750bb66efaea76d7ec4783320e6efdc45c468effb531c948a2292cL150-R150):
Removed unnecessary null-conditional operator when completing the
`Writer` of the event channel.
*
[`src/OpenFeature/FeatureProvider.cs`](diffhunk://#diff-96ebc8fc507d0a19d55b9a5cb57b72a0e8058e09f31ee7d0b39e99b00c5029d8L143-R143):
Made the `GetEventChannel` method non-virtual to ensure consistent
behavior across different implementations.

### Related Issues
<!-- add here the GitHub issue that this PR resolves if applicable -->

Related to #358

---------

Signed-off-by: André Silva <2493377+askpt@users.noreply.github.com>

Author: askpt

src/OpenFeature/EventExecutor.cs

@@ -147,7 +147,7 @@ private void StartListeningAndShutdownOld(FeatureProvider newProvider, FeaturePr
             if (oldProvider != null && !this.IsProviderBound(oldProvider))
             {
                 this._activeSubscriptions.Remove(oldProvider);
-                oldProvider.GetEventChannel()?.Writer.Complete();
+                oldProvider.GetEventChannel().Writer.Complete();
             }
         }
 

src/OpenFeature/FeatureProvider.cs

@@ -140,7 +140,7 @@ public virtual Task ShutdownAsync(CancellationToken cancellationToken = default)
         /// Returns the event channel of the provider.
         /// </summary>
         /// <returns>The event channel of the provider</returns>
-        public virtual Channel<object> GetEventChannel() => this.EventChannel;
+        public Channel<object> GetEventChannel() => this.EventChannel;
 
         /// <summary>
         /// Track a user action or application state, usually representing a business objective or outcome. The implementation of this method is optional.