Apply GitLab UI styles to buttons in app/views/shared/labels directory

What does this MR do?

Apply GitLab UI button styles to buttons in app/views/shared/labels directory

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. no additional test for this change
• Tested in all supported browsers
  • Chrome
  • Firefox
  • Chromium
  • Safari
  • Edge I don't have access to Safari or Edge
• 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 to this change_
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods Not applicable to this change
• Security reports checked/validated by a reviewer from the AppSec team Not applicable to this change

Closes #231209 (closed)