Update examples to use gazebosim.org

[nkoenig]

🎉 New feature

Summary

Updates examples to use gazebosim.org instead of ignitionrobotics.org. This is targeting citadel, and is part of the overall ignition to gazebo renaming.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 #1749 (1a61269) into ign-gazebo3 (f16b35e) will increase coverage by 0.04%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo3    #1749      +/-   ##
===============================================
+ Coverage        77.94%   77.98%   +0.04%     
===============================================
  Files              255      255              
  Lines            15041    15041              
===============================================
+ Hits             11723    11730       +7     
+ Misses            3318     3311       -7     

Impacted Files	Coverage Δ
src/Server.cc	84.93% <0.00%> (+0.60%)	⬆️
src/SimulationRunner.cc	94.51% <0.00%> (+1.06%)	⬆️

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