Add alert informing the user about new editing flow for Branch rules

Since we will be introducing a new flow of editing branch rules, I think it will be beneficial to inform users about it when we enable the feature flag. The branch rule details page was so far available as a read-only. It may be hard to discover for users that we added editing to it

Looking at our design system, I think this should be either Alert in Information or Tip variant:

Alert Information	Alert Tip

This could be a page-level alert on Settings / Repository, where the primary button will expand Branch rules section. It could also be reused on Settings / Merge requests / Merge requests approval rules as an in-page level alert.

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Title: Branch rules editing

Description: You can now edit all branch rule details from a single page. How to edit branch protections, merge request approval rules, and more?

Primary button: Edit branch rules

Secondary button: Dismiss

Documentation link: https://docs.gitlab.com/ee/user/project/repository/branches/#create-a-branch-rule

Additionally:

Remove this other alert that pushes people to branch rules when you create a protected branch the classic way (see #475215 (comment 2024295593))