Minor style changes for onboarding popovers

What does this MR do?

This adds minor fixes for the onboarding popovers introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13346 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13363

It adds the following changes:

• Increase button size in popovers from xs to sm
• Changes popover class name to onboarding-popover
• Utilize cssClasses prop of popover components for adding custom classes (so it adds onboarding-popover to the popover element itself)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team