Deprecate the missingCodeBlockLanguage warning.

lib/src/model/documentation_comment.dart

@@ -1,3 +1,9 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:args/args.dart';
 import 'package:crypto/crypto.dart' as crypto;
@@ -676,6 +682,8 @@ mixin DocumentationComment
     for (var element in firstOfPair) {
       final result = element.group(2)!.trim();
       if (result.isEmpty) {
+        stderr.writeln("The warning 'missingCodeBlockLanguage' is deprecated. "
+            'Use the `missing_code_block_language_in_doc_comment` lint instead.');
         warn(PackageWarning.missingCodeBlockLanguage,
             message:
                 'A fenced code block in Markdown should have a language specified');

lib/src/warnings.dart

@@ -279,15 +279,17 @@ enum PackageWarning implements Comparable<PackageWarning> {
     'deprecated dartdoc usage: {0}',
     shortHelp: 'A dartdoc directive has a deprecated format.',
   ),
+  // TODO(kallentu): Remove this warning.
   missingCodeBlockLanguage(
     'missing-code-block-language',
     'missing code block language: {0}',
-    shortHelp: 'A fenced code block is missing a specified language.',
-    longHelp:
-        'To enable proper syntax highlighting of Markdown code blocks, Dartdoc '
-        'requires code blocks to specify the language used after the initial '
-        'declaration. As an example, to specify Dart you would open the '
-        'Markdown code block with ```dart or ~~~dart.',
+    shortHelp: '(Deprecated: Use `missing_code_block_language_in_doc_comment` '
+        'lint) A fenced code block is missing a specified language.',
+    longHelp: '(Deprecated: Use `missing_code_block_language_in_doc_comment` '
+        'lint) To enable proper syntax highlighting of Markdown code blocks, '
+        'Dartdoc requires code blocks to specify the language used after the '
+        'initial declaration. As an example, to specify Dart you would open '
+        'the Markdown code block with ```dart or ~~~dart.',
     defaultWarningMode: PackageWarningMode.ignore,
   );