Fix NamespaceCache usage in System.Private.Xml.Linq (#101158)

It's intended to be mutated on Get so that it can cache the XNamespace object - the fields being marked readonly completely block all caching.

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XContainer.cs

@@ -880,8 +880,8 @@ internal async Task ReadContentFromAsync(XmlReader r, LoadOptions o, Cancellatio
 
         private sealed class ContentReader
         {
-            private readonly NamespaceCache _eCache;
-            private readonly NamespaceCache _aCache;
+            private NamespaceCache _eCache;
+            private NamespaceCache _aCache;
             private readonly IXmlLineInfo? _lineInfo;
             private XContainer _currentContainer;
             private string? _baseUri;