Docs feedback: Clarify set dedicated port part of Kerberos integration troubleshooting for "unsupported GSSAPI mechanism"

In the unsupported GSSAPI mechanism subsection of the Kerberos integration documentation, we list a few possible explanations for how to address this error:

OmniauthKerberosSpnegoController: failed to process Negotiate/Kerberos authentication: gss_accept_sec_context did not return GSS_S_COMPLETE: An unsupported mechanism was requested Unknown error

We offer:

1. Ensure that there is connectivity between the client machine and the Kerberos server
2. Ensure that the forward and reverse DNS records for the GitLab server match
3. Ensure that the Kerberos libraries are installed on the browser or client machine

In working with a customer on a ticket available to GitLab team members with access to Zendesk, we identified another possible solution for this error message. This solution is already mentioned on the Kerberos integration page but we don't mention using a dedicated port as one of the solutions in the unsupported GSSAPI mechanism troubleshooting subsection.

📋 Action Items for this Issue

• Add "ensure the Kerberos integration is configured to use a dedicated port to the unsupported GSSAPI mchanism troubleshooting subsection
• Consider cleaning up the unsupported GSSAPI mchanism troubleshooting subsection. The four available solutions should be put in order (try this one first) and made a bit clearer for the reader to identify.

📚 References

• #49