Update CI

[falbrechtskirchinger]

• Bump macos-10.5 runner to macos-11 and update Xcode versions
  https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md#xcode
  https://github.com/actions/virtual-environments/blob/main/images/macos/macos-12-Readme.md#xcode
  https://gist.github.com/yamaya/2924292#file-xcode-clang-vers

• Fix CodeQL workflow warning:

  2 issues were detected with this workflow: git checkout HEAD^2 is no longer necessary. Please remove this step as Code Scanning recommends analyzing the merge commit for best results. Please make sure that every branch in on.pull_request is also in on.push so that Code Scanning can compare pull requests against the state of the base branch.

• Only start AppVeyor builds if a commit touches these paths:

  cmake/
  include/
  tests/
  CMakeLists.txt
  

Fixes #3612.

[coveralls]

Coverage remained the same at 100.0% when pulling cea3a07 on falbrechtskirchinger:ci-update into 817a4a2 on nlohmann:develop.

[nlohmann]

Looks good to me.

[falbrechtskirchinger]

One more note for you:

xcode: ['11.7', '12.4', '12.5.1', '13.0']

13.0 turns into 13 and CI fails. Seems like good practice to use strings in cases like this in the future.

[nlohmann]

One more note for you:

xcode: ['11.7', '12.4', '12.5.1', '13.0']

13.0 turns into 13 and CI fails. Seems like good practice to use strings in cases like this in the future.

And that answers my unasked question why you added quotes around all the version numbers :-)