Hide user attributes behind a feature flag

What does this MR do and why?

There is unclarity in !95796 (merged) about whether removing all user attributes from both system notes and discussions is a good idea, so we could first test this behind a feature flag. That feature flag will only be enabled for gitlab-org and gitlab-com. If we don't find any new negative feedback that's not already covered in #372234 (closed), we will then merge !95796 (merged), which completely removes these attributes and adapts all tests.

Screenshots or screen recordings

Before	After

How to set up and validate locally

bin/rails c
Feature.enable(:remove_user_attributes)

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.