add describeEnum removal breaking change entry and migration guide

[AbdeMohlbi]

see flutter/flutter#190076 (comment)

Issues fixed by this PR (if any):

_PRs or commits this PR depends on (if any):flutter/flutter#190076

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[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 8c092de):

https://flutter-docs-prod--docs-pr13682-describeenum-removal-5t0l5ucm.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 8c092de):

https://flutter-dev-230821--www-pr13682-describeenum-removal-1by3zed8.web.app

[gemini-code-assist]

Code Review

This pull request documents the removal of the describeEnum method from the Flutter framework, which has been replaced by the native name getter on enums since Dart 2.14. The review feedback highlights several grammatical and formatting issues in the documentation text, as well as minor syntax errors and inconsistencies in the Dart code examples.