ConfigXmlReader is an IDisposable; Dispose it (#103163)

This was flagged by a static analysis tool. Is this necessary? Is this
safe to do?

src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/AppSettingsSection.cs

@@ -127,7 +127,7 @@ protected internal override void DeserializeElement(XmlReader reader, bool seria
                 }
             }
 
-            ConfigXmlReader internalReader = new ConfigXmlReader(rawXml, sourceFileFullPath, lineOffset);
+            using ConfigXmlReader internalReader = new ConfigXmlReader(rawXml, sourceFileFullPath, lineOffset);
             internalReader.Read();
 
             if (internalReader.MoveToNextAttribute())