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 removed the redundant ci-bionic workflow.

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 #126 into ign-fuel-tools4 will increase coverage by 0.31%.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           ign-fuel-tools4     #126      +/-   ##
===================================================
+ Coverage            76.80%   77.12%   +0.31%     
===================================================
  Files                   19       19              
  Lines                 2535     2535              
===================================================
+ Hits                  1947     1955       +8     
+ Misses                 588      580       -8     

Impacted Files	Coverage Δ
src/WorldIter.cc	100.00% <0.00%> (+8.98%)	⬆️

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