Move rule code from description to check_name in GitLab output serializer

[InSyncWithFoo]

Summary

Resolves #16435.

The changes are summarized by the following table:

Property	Has rule?	Before	After
check_name	Yes	-	A123
description	Yes	A123: Some explanation	Some explanation
check_name	No	-	syntax-error
description	No	SyntaxError: Some explanation	Some explanation

Test Plan

cargo nextest run and cargo insta test.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[InSyncWithFoo]

I don't use GitLab, so this is pretty much guesswork. The description of check_name says it should be "[a] unique name representing the check, or rule, associated with this violation". Is the rule code enough, or should it be more similar to that of --output-format=github (Ruff (A123))?

[MichaReiser]

The internal name seems fine. This is what eslint does

https://gitlab.com/remcohaszing/eslint-formatter-gitlab/-/blob/main/lib/eslint-formatter-gitlab.js?ref_type=heads#L121

[MichaReiser]

Thanks, this is great