Seal overridden virtual method SetDefaultFallbacks

Fix CA2214

Author: xtqqczze

src/libraries/System.Private.CoreLib/src/System/Text/Latin1Encoding.cs

@@ -26,7 +26,7 @@ public Latin1Encoding() : base(ISO_8859_1)
         public override ReadOnlySpan<byte> Preamble => default;
 
         // Default fallback that we'll use.
-        internal override void SetDefaultFallbacks()
+        internal sealed override void SetDefaultFallbacks()
         {
             // We use best-fit mappings by default when encoding.
             encoderFallback = EncoderLatin1BestFitFallback.SingletonInstance;

src/libraries/System.Private.CoreLib/src/System/Text/UTF32Encoding.cs

@@ -62,7 +62,7 @@ public UTF32Encoding(bool bigEndian, bool byteOrderMark, bool throwOnInvalidChar
                 SetDefaultFallbacks();
         }
 
-        internal override void SetDefaultFallbacks()
+        internal sealed override void SetDefaultFallbacks()
         {
             // For UTF-X encodings, we use a replacement fallback with an empty string
             if (_isThrowException)