Require gz-cmake 2.13

[chapulina]

• Follow up to Change IGN_DESIGNATION to GZ_DESIGNATION #468

Summary

Math won't work with lower versions now.

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.

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[codecov]

Codecov Report

Merging #474 (e7707ca) into ign-math6 (d4bbef8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #474   +/-   ##
==========================================
  Coverage      99.68%   99.68%           
==========================================
  Files             73       73           
  Lines           6913     6913           
==========================================
  Hits            6891     6891           
  Misses            22       22           

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