Add C++23 interpreter testing and workflow

[aaronj0]

Ensures that the C++23 tests are always run, and drops the workaround of using a separate interpreter for those tests.

[github-actions]

Test Results

Configuration	Result
macos-26-intel-llvm21-py3.14-cxx20	= 437 passed, 44 skipped, 85 xfailed, 32 xpassed, 100 warnings in 103.60s (0:01:43) =
macos-26-llvm21-py3.14-cxx20	==== 438 passed, 37 skipped, 93 xfailed, 30 xpassed, 100 warnings in 48.70s ====
ubuntu-24.04-llvm21-py3.14-cxx20-vg	====== 553 passed, 25 skipped, 18 xfailed, 2 xpassed in 91.62s (0:01:31) =======
ubuntu-24.04-llvm22-py3.14-cxx20	====== 553 passed, 25 skipped, 19 xfailed, 1 xpassed in 117.22s (0:01:57) ======
ubuntu-24.04-llvm22-py3.14-cxx20-cling	====== 543 passed, 25 skipped, 27 xfailed, 3 xpassed in 100.37s (0:01:40) ======
ubuntu-24.04-llvm22-py3.14-cxx23	====== 574 passed, 3 skipped, 20 xfailed, 1 xpassed in 104.04s (0:01:44) =======