Release v1.2.1

chore: bump the action image to v1.2.1

Ship a prebuilt multi-arch image from GHCR instead of building per run

fix: correct license badge to MIT, optimize Docker build

refactor: rename git-mirror-action to multi-git-mirror across entire …

…project

ci: disable Bitbucket tests until plan issue is resolved

- Remove Bitbucket from CI dry-run tests (402 write access error)
- Disable bitbucket-mirror.yml auto-trigger (keep workflow_dispatch)
- Multi-target tests use GitHub + GitLab only

fix: resolve parallel race condition, add mutex and prevent credentia…

…l prompts

- Add configMu mutex to protect concurrent git remote add/remove operations
- Set GIT_TERMINAL_PROMPT=0 to prevent interactive credential prompts
- Use git init -b main to suppress default branch hint messages
- Pass GITLAB_TOKEN for GitLab dry-run tests in CI
- Test both GitHub and GitLab providers in CI (dry-run only, no push)