chore(mix_generator): prepare 2.2.1 release

[leoafarias]

Related issue

Release preparation for #1087.

Description

Prepare mix_generator 2.2.1 with analyzer >=10.0.0 <15.0.0 support. This releases the merged analyzer 14 compatibility fix; generated runtime APIs remain unchanged.

Changes

• Bump mix_generator from 2.2.0 to 2.2.1 and finalize its changelog.
• Explicitly document that analyzer 9 is no longer supported; the minimum is 10.0.0.

Review Checklist

• Testing: All 380 generator tests pass on the release branch with analyzer 14.4.0. The merged implementation also passed all 380 tests with analyzer 10.0.0 during review. dart analyze reports no errors or warnings (four existing infos). Changelog verification and git diff --check pass. dart pub publish --dry-run reports zero warnings after commit.
• Breaking Changes: The minimum analyzer dependency increases from 9 to 10, as documented; generated runtime APIs are unchanged.
• Documentation Updates: Release changelog finalized.
• Website Updates: Not required for this package compatibility release.

Additional Information

Draft pending repository-wide CI. The prescribed local melos run gen:build && melos run ci && melos run analyze sequence could not start because the globally installed Melos snapshot is incompatible with the selected SDK and its automatic recovery could not complete in the restricted environment.

After merge and successful checks, publishing the tag mix_generator-v2.2.1 on the merged commit triggers the existing pub.dev workflow. No release tag or publication is included in this PR.