Update `messages.yaml`s language version overrides

[FMorschel]

We have some different messages.yaml files. These are the ones I could find/remember:

• pkg/analyzer/messages.yaml
• pkg/front_end/messages.yaml
• pkg/liner/messages.yaml
• pkg/analyzer/messages.yaml

This is coming from a comment on https://dart-review.googlesource.com/c/sdk/+/407080 from @bwilkerson:

This language marker doesn't appear to be necessary (pasting the example without it into an editor produces the expected diagnostic).

We should probably just get rid of all %language markers, replacing them with ``// dart=` comments where they're actually required. (But not as part of this CL.)

About places like these:

sdk/pkg/analyzer/messages.yaml

Line 1458 in 7349ef5

%language=2.18

That should become something like:

sdk/pkg/analyzer/messages.yaml

Line 4896 in 7349ef5

// @dart = 2.8

Because of INVALID_LANGUAGE_VERSION_OVERRIDE (if I'm not mistaken).

[dart-github-bot]

Summary: Multiple Dart SDK packages contain messages.yaml files with unnecessary language version overrides. These should be removed or replaced with // dart= comments where needed.