[mlir] Update Python CMake version requirement.

The minimum required version is now 3.19 due to the usage of some
more recent features. Update the version check and error message
accordingly. Also remove some logic that behaved differently before
3.18, since we can assume we are now on version 3.19+.

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D130171