Make ISessionSerializer internal (#573)

As part of getting changeset session state serialization working, we need to make changes to this interface. It's not what we expect people to use now for implementing session serialization customizations (instead use ISessionKeySerializer), so until we know what that will look like, it'll be internal.

Author: twsouthwick

src/Microsoft.AspNetCore.SystemWebAdapters.Abstractions/SessionState/Serialization/ISessionSerializer.cs renamed to src/Services/SessionState/ISessionSerializer.cs

@@ -7,7 +7,7 @@
 
 namespace Microsoft.AspNetCore.SystemWebAdapters.SessionState.Serialization;
 
-public interface ISessionSerializer
+internal interface ISessionSerializer
 {
     Task<ISessionState?> DeserializeAsync(Stream stream, CancellationToken token);