Adopt ifx, drop ifort

[wpbonelli]

Intel will not provide ifort downloads indefinitely. Some work on moving to ifx has begun. Compatibility has been tested for a number of versions. So far none have passed all tests out of the box, but at last investigation most or all remaining failures for several recent ifx versions were isolated to PRT and due to small numerical differences in results.

This should be confirmed and disagreements listed here and marked as they are checked/resolved.

By way of proximate motivation, it seems like we just lost ifort 2021.7 on macos-13 in CI

[wpbonelli]

Seems like an intel macos image will be available until mid-2027 which may relieve some of the near term pressure to migrate, but we still have no guarantee that intel will keep the old oneapi installers available. cf #2604