Add alert when converting runner to specific

Description

This issue is a follow-up for the changes being made in !68966 (merged). Since we are now adding an extra confirmation when restricting a runner to one or more projects, we should be confirming that those changes have been saved successfully/failed.

Proposal

Add the same Changes saved alert at the top of the page once the user confirms they want to change to a specific runner by selecting Ok. This is the same alert behavior we provide when Saving changes in general on that page.