Enable GitHub Actions for macos-11 runners

[kulp]

Since #85, and as of the closure of actions/runner-images#2486, macOS 11 Big Sur is generally available on GitHub-hosted runners and so we will start using macos-11 in our CI alongside macos-latest.

[codecov]

Codecov Report

Merging #96 (d63b97d) into develop (41d626f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop     #96   +/-   ##
=======================================
  Coverage     96.2%   96.2%           
=======================================
  Files           34      34           
  Lines         2667    2667           
  Branches       470     470           
=======================================
  Hits          2566    2566           
  Misses          77      77           
  Partials        24      24           

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