Add messaging to redirect users from Protected branches to Branch rules section

Context

&12494 (comment 2317268709) &17598

Now that Branch rules have been released, having the Repository Settings > Protected branches section may be unnecessary and even confusing to users.

Add messaging to redirect users from Protected branches to Branch rules.

No feature flags would be introduced as part of this work.

Design proposal

See #523300 (comment 2476950142)

Default	When Protected Branches section is open

Display duration

The alert should be displayed for 3 milestones. See #523300 (comment 2465741507).

#538383 will address the removal of this alert.