Remove Serverless from GitLab

What does this MR do and why?

Removes

• All user-facing Serverless features from GitLab
• All associated documentation for the feature

See

• #353901 (closed)
• gitlab-org/configure&6 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. Open a project that you own
2. Hover the Infrastructure sidebar menu item
3. There should not be any Serverless menu item present

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.