CI: add clang builds

[braindigitalis]

Adds clang 10-15 to CI and bumps up concurrency on macos from 2 to 3.
clang++ 15 has CORO, 10-14 do not, to test a wide range of configurations.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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