Add announcement + history for admin perms change

What does this MR do?

Now that the feature has landed (with very very MVC docs), let's step back and figure out the full docs ramifications. @jwoodwardgl provided useful info here that tipped me off that more changes were needed:

We've changed the protected ref access logic to prevent admin users from being able perform actions on protected refs unless they have been explicitly allowed to do so. These actions include pushing and merging into a protected branch, unprotecting a branch, and creating protected tags.

Here's what I see is needed:

1. Check the protected-branches page.
2. Check the protected-tags page.
3. Create a breaking-change announcement

Related issues

• Related to Remove admin override for ProtectedRef Access (!118532 - merged) which introduced the change
• Related to Update who can modify a protected branch (!117737 - merged) which did the first part of the docs
• Related to Inconsistent behaviour for instance administrat... (#12776 - closed)
• Related to Display of fields in Protected Tags admin UI page (#409894 - closed) which is the UX issue I filed because I got so frustrated by not being able to read the fields

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