Automatically label test merges

[WolfSkin0]

About The Pull Request

Important

This PR has not been tested comprehensively yet by me because doing that in GitHub actions is a massive pain and I knocked this out in an afternoon between other bits of work.

Runs a cron job that checks TGS against GH for test merges and appropriately (un)labels PRs.

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This is a new github actions workflow that runs once every hour and check TGS against GitHub to detect whether a PR has been testmerged or not, applying and removing the 'TestMerge' label as necessary.

We'll need to discuss whether we think having the workflow run once per hour is enough or whether we want to change that.
Aside from that, there's also the case of if we want to have 2-way sync (i.e. applying the label in GitHub causing the PR to get testmerged in TGS automatically.
And there's the question of if we want to fully clean up the code now or focus on just getting this out first and then do cleanup as part of a later cleanup pass for CI.

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Changelog

🆑WolfSkin

server: Added auto-labelling workflow for test merges
/:cl:

[Takykardi]

Checks out with the rest of the workflows, should be good for deployment.