Improve fork experience

[chapulina]

Our GitHub actions haven't been running correctly for forks. This should fix all of them:

CI

CI only on push means that PRs from forks aren't tested. Adding pull_request fixes forks, with the downside of triggering duplicated jobs for PRs coming from branches in this repo.

Also removed the codecov token for the Focal builds because we only need to upload coverage reports once, and this causes failures sometimes.

And renamed the file to match other libraries, since the file runs for both bionic and focal.

Labeler

Resolves gazebo-tooling/pr-collection-labeler#4.

Uses the new pull_request_target event.

Tested here: gazebosim/testing#14

Triage

Like above, uses the new pull_request_target event.

Tested here: gazebosim/testing#15

[codecov]

Codecov Report

Merging #94 into ign-msgs5 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-msgs5      #94   +/-   ##
==========================================
  Coverage      85.66%   85.66%           
==========================================
  Files              7        7           
  Lines            795      795           
==========================================
  Hits             681      681           
  Misses           114      114           

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