Update archive modals' buttons

What does this MR do and why?

This updates the archive/unarchive modals buttons:

• The archive modal's button's label was changed to Archive project and its variant was set to warning.
• The unarchive modal's button's label was changed to Unarchive project.

Note that the labels and variant have been changed to match the trigger button's.

Screenshots or screen recordings

Archive modal

Before	After

Unarchive modal

Before	After

How to set up and validate locally

1. Enable the bootstrap_confirmation_modals feature flag.

   Feature.enable(:bootstrap_confirmation_modals)

2. Go to any project's general settings.
3. In the Housekeeping section, click on Archive project and confirm the action.
4. Go back to the project's general settings.
5. In the Housekeeping section, click on Unarchive project.

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 #344178 (closed)