Minor fixes

[jlduran]

This is a multi-commit pull request, given the trivial nature of the individual commits (none of the changes warrant a CHANGELOG entry):

1. Update Gemfile.lock.
2. Bump actions/checkout version in the GitHub CI script.
3. Minor fix in CHANGELOG.md.
4. Update README.md file.
5. Remove support for Ruby 2.7 (EoL).
6. Bump Ruby version in Dockerfile.

[pama]

Maybe we should remove ruby 2.7 from the CI, since it is no longer supported.

[pama]

Thanks!