Fix reflection in MemoryCache tests to support both versions seen in .NET 9 RCs (#3085)

Author: msbw2

tests/Microsoft.Identity.Web.Test/CacheEncryptionTests.cs

@@ -79,9 +79,12 @@ private byte[] GetFirstCacheValue(MemoryCache memoryCache)
                 .GetType()
                 .GetField("_coherentState", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)!
                 .GetValue(memoryCache)!;
-            memoryCacheContent = (content1?
-                .GetType()
-                .GetProperty("EntriesCollection", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)
+            Type? content1Type = content1?.GetType();
+            // The internals of CoherentState seem to change between .NET 9 RCs
+            memoryCacheContent = ((
+                content1Type?.GetProperty("_entries", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic) ??
+                content1Type?.GetProperty("_stringEntries", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)
+                )
                 .GetValue(content1) as IDictionary)!;
 #else
             memoryCacheContent = (memoryCache