Allow patch release of fleetd via releaser script

[sgress454]

For #21396

Details

This PR updates the automated release cycle for Orbit desktop, so that it triggers based on a pushed tag rather than a pushed PR. This has the following benefits:

• The release can be based off of any branch, rather than always using main as the base, so we can safely do patch release of desktop without including in-progress code from main
• It brings the desktop release process more in line with the main Orbit release process -- both are now triggered by a tag push.

We still create a PR for the release, to include a changelog.

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• [ ] Manual QA must be performed in the three main OSs, macOS, Windows and Linux.
• [ ] Auto-update manual QA, from released version of component to new version (see tools/tuf/test).

Testing

To do -- will discuss with @lucasmrod

[sgress454]

@lucasmrod updated to make the changelog PR do a cherry pick to main as discussed.

[sgress454]

@lukeheath we need a final +1 on this 🙏