config: drop support for Heimdal

[bagder]

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.

[icing]

Can't have memory leaks...😏

[bagder]

@vszakats can I ask you to have a look and maybe fix, as I clearely botched cmake here trying to remove heimdal detection?

[vszakats]

I'll have a look.

[vszakats]

Having a try at it at: #18932

[vszakats]

CMake support is ready at the other PR.

Looking at ./configure, it first says ignore, then detects it anyway, and the build completes with Heimdal:

configure: WARNING: incompatible GSS library found but ignored (heimdal?)
checking for pkg-config... (cached) /usr/bin/pkg-config
checking for mit-krb5-gssapi options with pkg-config... no
checking if we can link against GSS-API library... yes

https://github.com/curl/curl/actions/runs/18353983224/job/52281120021?pr=18932#step:27:234

Made it an error via 32869e8