gh-36778: Remove Cygwin distro support
Removing Cygwin support from:
- build system,
- CI,
- SPKG configuration and installation scripts,
- Sage library.
Depends on #36779
URL: https://github.com/sagemath/sage/pull/36778
Reported by: Michael Orlitzky
Reviewer(s): Dima Pasechnik, Matthias Köppe, Michael Orlitzky, Tobias Diez
