ci: fix CI workflow startup failure and polish README

[BeiBaiNian]

What changes were made

1. Fix CI workflow startup failure (Apache org actions allowlist)

The CI workflow previously used two third-party actions that are not allowed in the apache organization:

• supercharge/mongodb-github-action@1.7.0
• shogo82148/actions-goveralls@v1

This caused the workflow to fail at startup ("Startup failure") before any test could run. Both have been removed:

• The MongoDB replica set (port 42069, required by transaction tests) is now started directly with the official mongo:4.4 image via docker run + rs.initiate(), no third-party action needed.
• Coverage upload has been dropped (the corresponding dead badge was also removed from README).
• actions/checkout and actions/setup-go upgraded from v2 to v4 (official GitHub actions, allowlisted).

2. Fix release workflow bugs

• The "Write release notes" step only printed to stdout and never wrote release-notes.md, but the next step referenced it via body_path — the release would have failed on every tag push. Release notes are now written to the file.
• Replaced softprops/action-gh-release@v2 (also not guaranteed on the Apache allowlist) with the built-in gh CLI, which is idempotent (re-running safely updates the existing release).

3. Polish README

• Added the standard Apache License header (required by the ASF RAT check).
• Restructured the badge row: title on its own line, badges on the second line, aligned with the apache/casbin main repo style.
• All badges now point to apache/casbin-mongodb-adapter instead of the old casbin/mongodb-adapter links.

Checklist

• CI workflow passes the Apache org actions allowlist (only official actions/* used)
• release-notes.md is actually generated before the release step
• README has the Apache license header and valid badges

[hsluoyz]

done