Fix button variant in maintenance mode settings

What does this MR do and why?

This changes the variant of the Save changes button in the maintenance mode settings to confirm because it’s the primary action variant.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to Admin area
2. Settings > General
3. Expand the Maintenance mode section

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #347044 (closed)