Minor improvements to bump and tag actions

[alcaeus]

This PR contains a few changes to the bump-version and tag-commit actions:

• Changed the default tag message to "Release ${VERSION}" instead of "BUMP ${VERSION}" - I assumed the latter was wrong as it's the same as the bump commit
• Changed output style of the "created commit/tag" messages to no longer appear as headings in the build output
• Made the version_bump_script input in bump_version optional - this can be useful to skip the bump step if it happens before (as is the case in PHP where we bump earlier to sanity-check the input before starting the release process)
• (nit) Cleaned up style of variable usage

[blink1073]

LGTM!