[ci] Drop cppyy support and testing with LLVM18

[aaronj0]

No description provided.

[mcbarton]

Thanks. Please open up PRs in all the cppyy repos to drop the llvm 18 jobs, and add me as a reviewer

[aaronj0]

We should first land compiler-research/cppyy#172 and rerun the CI here to ensure the effects were correct.

[aaronj0]

Thanks. Please open up PRs in all the cppyy repos to drop the llvm 18 jobs, and add me as a reviewer

Will do, thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.32%. Comparing base (22bfddc) to head (90e4ccf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #788   +/-   ##
=======================================
  Coverage   79.32%   79.32%           
=======================================
  Files          11       11           
  Lines        3966     3966           
=======================================
  Hits         3146     3146           
  Misses        820      820           

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