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._