build: drop Heimdal support, update docs, replace with MIT Kerberos in CI

build: drop Heimdal support, update docs, replace with MIT Kerberos in CI

The kerberos5 library Heimdal is one of three GSS libraries curl support.
It has a memory leak triggered by the new test in #18917 and the project
seems mostly abandoned.

Drop support and steer users to the MIT krb5 or GNU GSS libraries.

Co-authored-by: Daniel Stenberg
Ref: #18928
Closes #18928
Closes #18932