Fix CXX_STANDARD in core library

[scpeters]

🦟 Bug fix

Fixes .pc file (see osrf/homebrew-simulation#2016 (comment))

Summary

We used to use a c++standard cmake variable, but recently changed to using CMAKE_CXX_STANDARD but hadn't updated the CXX_STANDARD argument to gz_create_core_library. This caused a pkg-config compilation test to fail during a brew bottle build, but it should be fixed now.

Confirm by checking for a -std=c++17 flag in the output of pkg-config --cflags gz-utils2.

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.

[codecov]

Codecov Report

Merging #77 (1cf5737) into gz-utils2 (d8d6c91) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           gz-utils2      #77   +/-   ##
==========================================
  Coverage      91.97%   91.97%           
==========================================
  Files              6        6           
  Lines            137      137           
==========================================
  Hits             126      126           
  Misses            11       11           

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