Resolve "Apply GitLab UI button styles to buttons in app/views/admin/deploy_keys directory"

What does this MR do?

This adds the gl-button class to the various buttons in the following files:

• app/views/admin/deploy_keys/edit.html.haml
• app/views/admin/deploy_keys/index.html.haml
• app/views/admin/deploy_keys/new.html.haml

For the new.html.haml and edit.html.haml files, where classes were listed as btn-success btn, they have been switched to follow the order of class listings in other areas.

Screenshots

Unavailable - unable to access admin area as part of a Gitpod instance

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) Not applicable for this change
• Code review guidelines Not applicable for this change
• Merge request performance guidelines Not applicable for this change
• Style guides Not applicable for this change
• Database guides Not applicable for this change
• Separation of EE specific content Not applicable for this change

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process. Not applicable for this change
• Tested in all supported browsers
  • Note I have been unable to test as the GitPod instance does not provide access to the /admin route as described in the docs from what I can see.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done Not applicable for this change

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~ Not applicable for this change
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods Not applicable for this change
• Security reports checked/validated by a reviewer from the AppSec team Not applicable for this change

Related to #231214 (closed)