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 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 #54 into ign-sensors3 will increase coverage by 15.72%.
The diff coverage is 60.00%.

@@                Coverage Diff                @@
##           ign-sensors3      #54       +/-   ##
=================================================
+ Coverage         61.88%   77.61%   +15.72%     
=================================================
  Files                13       23       +10     
  Lines               942     2296     +1354     
=================================================
+ Hits                583     1782     +1199     
- Misses              359      514      +155     

Impacted Files	Coverage Δ
include/ignition/sensors/Manager.hh	90.00% <ø> (ø)
src/GaussianNoiseModel.cc	98.30% <ø> (+42.12%)	⬆️
src/Manager.cc	81.08% <ø> (+43.34%)	⬆️
src/DepthCameraSensor.cc	74.05% <33.33%> (ø)
src/Noise.cc	73.33% <33.33%> (-1.67%)	⬇️
src/RgbdCameraSensor.cc	74.56% <33.33%> (ø)
src/ImageNoise.cc	43.47% <43.47%> (ø)
src/CameraSensor.cc	74.81% <52.63%> (ø)
src/ThermalCameraSensor.cc	57.01% <57.01%> (ø)
src/GpuLidarSensor.cc	88.31% <66.66%> (ø)
... and 28 more

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[j-rivero]

@osrf-jenkins run tests please