Label workflow maintenance

[dcamron]

• Reduce cron schedule to weekly Mondays 1200 UTC. I don't see any reason this needs to be hourly. We can probably even only change this to on push to labels.yml contents sometime, but wanted to leave this active while we were iterating and until we change up the workflow.

• Actions relying on node 12 (eg checkout@v2) will be deprecated. Manually bump this and we'll see if it works. I'll manually test this after merge.

Re-generated my PAT in the repo settings, so we should be good until July. I'll close #24 once confirmed this works

[dcamron]

@ProjectPythia/infrastructure no auto review assignment here, please take a look and choose to review, else I can pick on someone at our next meeting

[brian-rose]

@dcamron thanks for this update! Let's merge this and, assuming it's working as it should, I can open another PR to add other repos to the list that gets synced.

[dcamron]

@brian-rose this seems to be working correctly, but it's hard to tell without making any labels changes.