Skip GitHub Release but maintain Git tags for future version & changelog generation

Is your feature request related to a problem? Please describe.

The ability to disable GitHub releases while still maintaining git tags so that changelogs and version increments for packages can be maintained.

Describe the solution you'd like

For the skip-github-release to still create Git tags, but just skip the GitHub Release portion as suggested by the flag name.

https://github.com/googleapis/release-please/blob/7584fecd34794ed146117c80a7f37f8e64f9d0bf/src/strategies/base.ts#L381-L384

Describe alternatives you've considered

Another solution could be to create a new option for skip-git-tag to work alongside skip-github-release for more granular control over git tags vs. GitHub Releases.

Additional context

@chingor13: The skip-github-release option only absolves release-please of its duty to handle the tagging of releases on GitHub. It still needs to know what the SHA of the last release of that component was (so we can determine commits since last release).

Discovered in #1249.