github-ci: consolidate new authors check into a single workflow

[jasonish]

Replace the two-workflow pattern (authors.yml + authors-done.yml) with a
single pull_request_target workflow.

I'm not sure this was possible when this job was originally created, but
apparently it is now.

Why: We recently saw the new authors check run and find new authors, but the second workflow to add the comment didn't run. And this is just simpler.

[jasonish]

This is hard to test, as it has to exist on main. I tested on my own repo and the message can be seen here: jasonish#328

But to really test, someone needs to create a pull request against my repo, where the source repo is not my repo as well. And use a new ID that has not been seen before.

[suricata-qa]

Information: QA skipped due to no C or rust code changed detected. Set to force a run.

Pipeline = code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.85%. Comparing base (0eda09f) to head (0491ac9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15571      +/-   ##
==========================================
- Coverage   82.86%   82.85%   -0.01%     
==========================================
  Files         999      999              
  Lines      272627   272627              
==========================================
- Hits       225904   225884      -20     
- Misses      46723    46743      +20     

Flag	Coverage Δ
fuzzcorpus	61.48% <ø> (ø)
livemode	18.40% <ø> (+<0.01%)	⬆️
netns	22.78% <ø> (-0.05%)	⬇️
pcap	45.20% <ø> (-0.01%)	⬇️
suricata-verify	66.56% <ø> (-0.01%)	⬇️
unittests	58.48% <ø> (ø)

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[victorjulien]

replaced by #15598