macOS AppleClang12 Fixes

[ax3l]

CI: macOS-10.15

CI already runs on macOS-12. Adds an older runner.
Needs to update to macOS-11 soon.

Fix: Older libc++ (e.g. AppleClang, LLVM Clang <7)

    In file included from /Users/runner/work/openPMD-api/openPMD-api/src/include/openPMD/RecordComponent.hpp:467:
    /Users/runner/work/openPMD-api/openPMD-api/src/include/openPMD/RecordComponent.tpp:381:20: error: no matching constructor for initialization of 'std::shared_ptr<char []>'
                return std::shared_ptr< T[] >{ new T[ size ] };
                       ^                     ~~~~~~~~~~~~~~~~~

Issue seen with AppleClang 12.0 and not seen with 14.0.
Assume 13 might be affected, too.

[ax3l]

tested here with new CI runner and in #1392 ✔️