ci: speed up ci a bit by thinning out matrix

[henryiii]

Also modernizing slightly. We should probably get off of ubuntu-20.04 as it's probably not going to be a runner for much longer.

[rwgk]

Looks good to me. Do you still want to get rid of the job with -DPYBIND11_INTERNALS_VERSION=10000000? Currently it's pretty much useless, as you suspected. I think you can simply delete this block:

• pybind11/.github/workflows/ci.yml

  Lines 203 to 219 in a28ea8c

  	# Third build - C++17 mode with unstable ABI
  	- name: Configure (unstable ABI)
  	run: >
  	cmake -S . -B build3
  	-DPYBIND11_WERROR=ON
  	-DPYBIND11_PYTEST_ARGS=-v
  	-DDOWNLOAD_CATCH=ON
  	-DDOWNLOAD_EIGEN=ON
  	-DCMAKE_CXX_STANDARD=17
  	-DPYBIND11_INTERNALS_VERSION=10000000
  	${{ matrix.args }}
  	- name: Build (unstable ABI)
  	run: cmake --build build3 -j 2
  	- name: Python tests (unstable ABI)
  	run: cmake --build build3 --target pytest