[ign to gz] Deprecate IGN_GUI_PLUGIN_PATH in favor of GZ_GUI_PLUGIN_PATH

[chapulina]

🎉 New feature

• Part of Rename Ignition to Gazebo gazebo-tooling/release-tools#698

Summary

Rename the environment variable with a tick-tock cycle.

Test it

Try out the hello_pluginin examples/plugin/hello_plugin:

$ GZ_GUI_PLUGIN_PATH=`pwd`/build ign gui -s HelloPlugin -v 2
# Loads the plugin without warnings.

$ IGN_GUI_PLUGIN_PATH=`pwd`/build ign gui -s HelloPlugin -v 2
[GUI] [Wrn] [Application.cc:454] Found plugin [HelloPlugin] using deprecated environment variable [IGN_GUI_PLUGIN_PATH]. Please use [GZ_GUI_PLUGIN_PATH] instead.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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.

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[codecov]

Codecov Report

Merging #384 (af46151) into main (966c73a) will decrease coverage by 0.00%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
- Coverage   64.12%   64.11%   -0.01%     
==========================================
  Files          45       45              
  Lines        5508     5518      +10     
==========================================
+ Hits         3532     3538       +6     
- Misses       1976     1980       +4     

Impacted Files	Coverage Δ
src/Application.cc	84.82% <71.42%> (-0.70%)	⬇️

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[jennuine]

LGTM, one minor comment