docs/DEPRECATE.md: deprecate gskit

Ref: #10163

- 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"

Closes #10201

docs/DEPRECATE.md

@@ -40,6 +40,18 @@ We remove support for building curl with the NSS TLS library in August 2023.
 Starting in 7.82.0, building curl to use NSS configure requires the additional
 flag `--with-nss-deprecated` in an attempt to highlight these plans.
 
+## gskit
+
+We remove support for building curl with the gskit TLS library in August 2023.
+
+- 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"
+
 ## past removals
 
  - Pipelining