Document that as a support team we don't support Apache

It is not clear from our documentation that Apache is not supported by our Support Engineering team, but is technically compatible.

It might come as a complete surprise to customers like this https://gitlab.zendesk.com/agent/tickets/72214 when we tell them that Apache is not supported. Which could be especially painful if they are a long way down the road of running Apache with GitLab in production.

This page make Apache look like a first class citizen: https://docs.gitlab.com/ee/development/architecture.html#gitlab-architecture-overview

This page details an integration for which Apache is required: https://docs.gitlab.com/ee/integration/shibboleth.html#shibboleth-omniauth-provider

My suggestion is:

1. Remove Apache from places where it is optional.
2. Put a disclaimer in places where Apache needs to be mentioned.
3. Put a Strong disclaimer or even consider removing things from the docs that require Apache.

/cc @seanpackham @lbot @stanhu