Add deprecation notice and links (#9221)

Fixes #9220

CustomBuildEventArgs is deprecated. Users are advised to use ExtendedCustomBuildEventArgs instead.
See #8917

The comment change results in a public-facing doc change at
https://learn.microsoft.com/en-us/dotnet/api/microsoft.build.framework.custombuildeventargs?view=msbuild-17-netcore

The new Remarks section includes a link to a public-facing doc that explains the change:
https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/custombuildeventargs

src/Framework/CustomBuildEventArgs.cs

@@ -10,12 +10,10 @@ namespace Microsoft.Build.Framework
     /// <summary>
     /// Arguments for custom build events.
     /// </summary>
-    // WARNING: marking a type [Serializable] without implementing
-    // ISerializable imposes a serialization contract -- it is a
-    // promise to never change the type's fields i.e. the type is
-    // immutable; adding new fields in the next version of the type
-    // without following certain special FX guidelines, can break both
-    // forward and backward compatibility
+    /// <remarks>
+    /// In MSBuild 17.8 and later, this type is deprecated; instead use <see cref="ExtendedCustomBuildEventArgs"/>.
+    /// For more information, see <see href="https://learn.microsoft.com/dotnet/core/compatibility/sdk/8.0/custombuildeventargs"/>
+    /// </remarks>
     [Serializable]
     public abstract class CustomBuildEventArgs : LazyFormattedBuildEventArgs
     {