Revert "Merge branch 'tor/maintenance/simplify-mr-header' into 'master'"

What does this MR do?

Revert "Merge branch 'tor/maintenance/simplify-mr-header' into 'master'"

This reverts merge request !138822 (merged)

Why?

Accessibility

There has been a significant downgrade in accessibility (see !138822 (comment 1699254398) and #429852 (comment 1699121753))

With a screenreader:

User X requested to merge Branch Y into Target Z

Reads now as:

User X Branch Y Target Z

So everything has lost it's meaning, I don't know what is a branch, what is a person. As someone who doesn't even use Voice Over, the dangling date is also weird. Is it created, updated, ???

Code dropdown

The dropdown is now inconsistent between merge requests and the project overview page (not as important as accessibility but maybe still worth noting). The goal in the original issue states:

Change the "Code" button which is currently the primary variant to the secondary variant so it doesn't clash with the merge button

The merge button is dependent on several factors though, whereas the code dropdown always shows up on the page. Even within GitLab, the majority of team members never see the Merge button on any MR in gitlab-org/gitlab (unless they're targeting a different branch). For those who do have maintainer+ permissions, that merge button still doesn't show up until approvals have been given, pipeline has passed, no conflicts, etc etc.

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.