Resolve "Apply GitLab UI button styles to buttons in app/views/projects/project_members directory"

What does this MR do?

This adds the gl-button class to the buttons on the project members import screen. This is as required by Epic &4001 (closed)

Screenshots

Before:

After:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• 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
  • Chromium
  • Chrome
  • Firefox
  • Safari
  • Edge
    Unable to test in Edge or Safari
• 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

Closes #231072 (closed)