Update CI

[awvwgk]

Dropping broken versions of GCC on MacOS.

• GCC 5 is not installable anymore on MacOS

  brew install gcc@5
  gcc@5: This formula either does not compile or function as expected on macOS
  versions newer than High Sierra due to an upstream incompatibility.
  Error: gcc@5: An unsatisfied requirement failed this build.
  

• gcov on MacOS is currently shaky with meson

   Traceback (most recent call last):
    File "/Users/runner/hostedtoolcache/Python/3.8.12/x64/bin/gcovr", line 8, in <module>
      sys.exit(main())
    File "/Users/runner/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/gcovr/__main__.py", line 280, in main
      error_occurred = print_reports(covdata, options, logger)
    File "/Users/runner/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/gcovr/__main__.py", line 432, in print_reports
      if generator(covdata, output.abspath, options):
    File "/Users/runner/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/gcovr/writer/html.py", line 397, in print_html_report
      max_line_from_cdata = max(cdata.lines.keys())
  ValueError: max() arg is an empty sequence
  Traceback (most recent call last):
  

• GCC 6 is broken on MacOS

  /usr/local/bin/gcc   -isysroot /Applications/Xcode_13.2.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.1.sdk -mmacosx-version-min=11.6 -o CMakeFiles/cmTC_09a2b.dir/testCCompiler.c.o -c /Users/runner/work/nlopt-f/nlopt-f/_build/CMakeFiles/CMakeTmp/testCCompiler.c
  <built-in>: error: unknown value '11.6' of -mmacosx-version-min
  

[codecov]

Codecov Report

Merging #14 (68447a1) into main (696b0e2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   29.48%   29.48%           
=======================================
  Files           8        8           
  Lines         563      563           
  Branches      153      153           
=======================================
  Hits          166      166           
  Misses        274      274           
  Partials      123      123           

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