Add missing_code_block_language_in_doc_comment lint. (#145354)

Adds this Dartdoc lint to the flutter repository, in replacement of the
warning it used to have.

Lint Proposal: https://github.com/dart-lang/linter/issues/4904

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Data Driven Fixes]:
https://github.com/flutter/flutter/wiki/Data-driven-Fixes

analysis_options.yaml

@@ -122,7 +122,7 @@ linter:
     # - lines_longer_than_80_chars # not required by flutter style
     - literal_only_boolean_expressions
     # - matching_super_parameters # blocked on https://github.com/dart-lang/language/issues/2509
-    # - missing_code_block_language_in_doc_comment # not yet tested
+    - missing_code_block_language_in_doc_comment
     - missing_whitespace_between_adjacent_strings
     - no_adjacent_strings_in_list
     - no_default_cases

dartdoc_options.yaml

@@ -40,4 +40,3 @@ dartdoc:
     ## Ignores that are elevated to errors:
     # - type-as-html # broken, https://github.com/dart-lang/dartdoc/issues/3545
     - missing-constant-constructor
-    - missing-code-block-language