release: prepare analytics 2.12.0

[github-actions]

Release analytics 2.12.0

This PR prepares the analytics module for release.

Changes

• Bumps version to 2.12.0 in packages/mixpanel_flutter/pubspec.yaml and any version_files listed in .github/modules.json
• Updates packages/mixpanel_flutter/CHANGELOG.md with a new section since the last v* tag
• Updates packages/mixpanel_flutter/README.md install snippet and version header

After merging

1. Push tag v2.12.0 from the merge commit on main to trigger the publish workflow:

   git checkout main && git pull
   git tag v2.12.0
   git push origin v2.12.0
   

2. The publish workflow creates a draft GitHub release and publishes to pub.dev. Review and publish the GitHub release after the workflow finishes.