Improve PTB nox task release:trigger

[ckunki]

PTB's Nox session release:trigger calls gh release list in one place for checking whether the current release potentially already has been published to GitHub.

The current ticket proposes removing that.

Rationale:

• Command gh requires GitHub credentials, while all the other commands do not.
• Also, in the past when simply using git tag $TAG; git push origin $TAG GitHub credentials have not been required, either.
• Additionally, GitHub releases are usually based on git tags, hence if PTB already successfully checked that there is no related tag, the additional check for the GitHub release to exist could be skipped.

However: PTB should call git fetch --all in order to also list remote tags, which currently might not be the case.

Acceptance Criteria

• Command gh release list is removed
• Command git fetch --all is added before git tag --list
• Tests are fixed / added