GitLab CI/CD does not work with Kerberos auth mode

What does this MR do?

Documents that GitLab CI/CD may not be an option if electing to enable Kerberos based HTTP authentication over a GitLab instance

• GitLab Runners require use of HTTP authentication
• When Kerberos is enabled, HTTP authentication responses include a Negotiate option
• Git picks up on this and begins attempting to use SPNEGO authentication at the runners
• Git lacks some changes required to fall back to job token authentication when SPNEGO (naturally) fails to work
• As a result, CI/CD jobs begin to always fail with authentication issues even when the right credentials are passed

It could theoretically be made to work if the ports are split for Kerberos, but this involves different URLs for different types of users/services that complicate the use of GitLab and likely also do not work for submodule accesses.

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/233244

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