Made ConvertName sealed

Co-authored-by: Eirik Tsarpalis <[email protected]>

Author: YohDeadfall

src/libraries/System.Text.Json/Common/JsonSeparatorNamingPolicy.cs

@@ -14,7 +14,7 @@ internal abstract class JsonSeparatorNamingPolicy : JsonNamingPolicy
         internal JsonSeparatorNamingPolicy(bool lowercase, char separator) =>
             (_lowercase, _separator) = (lowercase, separator);
 
-        public override string ConvertName(string name)
+        public sealed override string ConvertName(string name)
         {
             // Rented buffer 20% longer that the input.
             int bufferLength = (12 * name.Length) / 10;