Fix compiler warnings (CMP0072 and copy elision)

[chapulina]

🦟 Bug fix

Summary

We've been having CMP0072 policy warnings on Jenkins CI since we upgraded to Focal, see

• Default to Focal for CI gazebo-tooling/release-tools#565
• CMake policy CMP0072 gz-rendering#360

This PR takes the same approach as gazebosim/gz-rendering#528 to silence the CMP0072 warning.

I also tackled this other warning that I see locally with gcc 9.3, but is not coming up on CI yet:

/home/chapulina/dev_focal/ws_citadel/src/ign-sensors/src/SensorFactory.cc:140:19: warning: moving a local object in a return statement preve
nts copy elision [-Wpessimizing-move]            
  140 |   return std::move(result);                                                                                                         
      |          ~~~~~~~~~^~~~~~~~  

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸

[codecov]

Codecov Report

Merging #188 (c419ea6) into ign-sensors3 (d600490) will not change coverage.
The diff coverage is 0.00%.

@@              Coverage Diff              @@
##           ign-sensors3     #188   +/-   ##
=============================================
  Coverage         77.48%   77.48%           
=============================================
  Files                23       23           
  Lines              2367     2367           
=============================================
  Hits               1834     1834           
  Misses              533      533           

Impacted Files	Coverage Δ
src/SensorFactory.cc	54.43% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d600490...c419ea6. Read the comment docs.

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-04-13-fortress-edifice/1367/1