Implement RFC 20: Deprecate and remove non-FWFT FIFOs

[whitequark]

RFC: https://amaranth-lang.org/rfcs/0020-deprecate-non-fwft-fifos.html

• Deprecate non-FWFT FIFOs: Implement RFC 20: Deprecate non-FWFT FIFOs. #945
• Remove non-FWFT FIFOs: (Merge after 0.4 is released) Remove non-FWFT FIFOs #949
  • Changelog entry for removal