Install python3-gz-math7 instead of ignition

[chapulina]

🦟 Bug fix

Summary

That ignition there messes up with the sed command we have searching for gz in the install tutorial:

https://github.com/gazebosim/docs/blob/master/garden/install_ubuntu_src.md#install-dependencies

Checklist

• Signed all commits for DCO
• Added tests
• 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.

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[codecov]

Codecov Report

Merging #1092 (1d95bca) into main (e4f63fb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1092   +/-   ##
=======================================
  Coverage   83.10%   83.10%           
=======================================
  Files         155      155           
  Lines       18767    18767           
=======================================
  Hits        15596    15596           
  Misses       3171     3171           

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