Implement RFC 19: Remove `amaranth.lib.scheduler`

[whitequark]

RFC: https://amaranth-lang.org/rfcs/0019-remove-scheduler.html

• Deprecate amaranth.lib.scheduler: Implement RFC 19: Remove amaranth.lib.scheduler #888
• Remove amaranth.lib.scheduler: (Merge after 0.4 is released) Remove amaranth.lib.scheduler #961
  • Changelog entry for removal