Don't mutating the ScopeId on static readonly IPAddress fields (#115456)

The Address field was made readonly in dotnet/corefx#33531. Somehow ScopeId,
the only other mutatable property, was missed. I did not see discussion of
this in that PR or the related issue.

An example program that illistrates the problem:

```csharp
Console.WriteLine(IPAddress.IPv6Loopback);
IPAddress.IPv6Loopback.ScopeId = 1;
Console.WriteLine(IPAddress.IPv6Loopback);
```

This prints:

```txt
::1
::1%1
```

I think the right behavior is to throw when setting the ScopeId (this pr).

Contributes to #27870

Author: AustinWise

src/libraries/System.Net.Primitives/src/System/Net/IPAddress.cs

@@ -28,11 +28,11 @@ public class IPAddress : ISpanFormattable, ISpanParsable<IPAddress>, IUtf8SpanFo
 
         internal const uint LoopbackMaskHostOrder = 0xFF000000;
 
-        public static readonly IPAddress IPv6Any = new IPAddress((ReadOnlySpan<byte>)[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], 0);
-        public static readonly IPAddress IPv6Loopback = new IPAddress((ReadOnlySpan<byte>)[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1], 0);
+        public static readonly IPAddress IPv6Any = new ReadOnlyIPAddress([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], 0);
+        public static readonly IPAddress IPv6Loopback = new ReadOnlyIPAddress([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1], 0);
         public static readonly IPAddress IPv6None = IPv6Any;
 
-        private static readonly IPAddress s_loopbackMappedToIPv6 = new IPAddress((ReadOnlySpan<byte>)[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 255, 127, 0, 0, 1], 0);
+        private static readonly IPAddress s_loopbackMappedToIPv6 = new ReadOnlyIPAddress([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 255, 127, 0, 0, 1], 0);
 
         /// <summary>
         /// For IPv4 addresses, this field stores the Address.
@@ -438,6 +438,11 @@ public long ScopeId
                     ThrowSocketOperationNotSupported();
                 }
 
+                if (this is ReadOnlyIPAddress)
+                {
+                    ThrowSocketOperationNotSupported();
+                }
+
                 // Consider: Since scope is only valid for link-local and site-local
                 //           addresses we could implement some more robust checking here
                 ArgumentOutOfRangeException.ThrowIfNegative(value);
@@ -772,6 +777,9 @@ private sealed class ReadOnlyIPAddress : IPAddress
         {
             public ReadOnlyIPAddress(ReadOnlySpan<byte> newAddress) : base(newAddress)
             { }
+
+            public ReadOnlyIPAddress(ReadOnlySpan<byte> address, long scopeid) : base(address, scopeid)
+            { }
         }
     }
 }

src/libraries/System.Net.Primitives/tests/FunctionalTests/IPAddressTest.cs

@@ -346,6 +346,10 @@ public static void Address_ReadOnlyStatics_Set_Failure()
             Assert.Throws<SocketException>(() => IPAddress.Broadcast.Address = MaxAddress - 1);
             Assert.Throws<SocketException>(() => IPAddress.Loopback.Address = MaxAddress - 1);
             Assert.Throws<SocketException>(() => IPAddress.None.Address = MaxAddress - 1);
+
+            Assert.Throws<SocketException>(() => IPAddress.IPv6Any.ScopeId = 1);
+            Assert.Throws<SocketException>(() => IPAddress.IPv6Loopback.ScopeId = 1);
+            Assert.Throws<SocketException>(() => IPAddress.IPv6None.ScopeId = 1);
         }
 #pragma warning restore 618
     }