We remove support for building curl with gskit.
- This is a niche TLS library, only running on some IBM systems
- no regular curl contributors use this backend
- no CI builds use or verify this backend
- gskit, or the curl adaption for it, lacks many modern TLS features
making it an inferior solution
- build breakages in this code take weeks or more to get detected
- fixing gskit code is mostly done "flying blind"
This removal has been advertized in DEPRECATED in Jan 2, 2023 and it has
been mentioned on the curl-library mailing list.
It could be brought back, this is not a ban. Given proper effort and
will, gskit support is welcome back into the curl TLS backend family.
Closes#11460
78d6232f1f