Deprecate the missingCodeBlockLanguage warning.

[kallentu]

Deprecates the missingCodeBlockLanguage warning. This will be replaced by the missing_code_block_language_in_doc_comment lint.

This is a part of the effort to move more Dartdoc logic to the analyzer/linter.

See lint proposal here: dart-lang/linter#4904

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[kallentu]

@srawlins Is there anything else we need to mark for warning deprecations?

[srawlins]

Hmm it would be good to report the warning as deprecated, if enabled. It looks like that might be here (for enabling as an error) and here (for enabling as a warning).

If you add a _isDeprecated field to PackageWarning, then it should be pretty easy to check.

However, what does "report" mean? In our advanced warnings system, this is the code that sets up whether various checks are "warning" or "ignore" or "error", so it seems like you'd run into problems quick if you tried to report a warning about enabling a warning while setting up warnings. 😟 So... maybe you can just print to stderr or stdout and call it a day. 😀

[kallentu]

Yeah it seemed odd to report on a warning report.
Anyways, added a stderr, feel free to take a look