Add branch validation checks to release script

[oschwald]

Add safety checks to prevent broken releases:

• Check that releases are not run directly on the main branch
• Check that the release branch contains all commits from origin/main

Fixes ENG-3512

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)