Update border radius of form controls and remove space above page titles

What does this MR do?

Two things:

1. Removes the extra whitespace above page titles
2. Updates the border-radius of all .form-control elements from 2px to 4px.

The intention of these changes are to bring the product closer to compliance with the GitLab Design system.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57732

GitLab EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9152

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer