Confirmation notice a runner was assigned to a project does not have correct styles

Description

Note: This issue is blocked by Improve assigned projects section in edit view ... (#360826). Now that styles for flash usages in controllers are fixed this can be done.

In !70151 (merged), we added multiple confirmation alerts when assigning or unassigning runners from projects. Because we are still using the old backend controller to save the changes, we don't have much control over the appearance of the alert, so we had to use the informational style rather than success.

Proposal

We should update the alert so it includes:

• Success styling from https://design.gitlab.com/components/alert for when the runner was successfully assigned or unassigned from a project
• Danger styling from https://design.gitlab.com/components/alert for when there is an error when attempting to assign or unassign a runner from a project.

That way it will be more consistent with how we are using successful alerts within Runner details and other areas.