Update e2e selectors and page objects for diffs

What does this MR do?

• Removes deprecated qa classes for e2e diff and related selectors in favor of data-qa-selectors. (Part of #322769 (closed))
• Conforms selectors to naming conventions. (https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/doc/development/testing_guide/end_to_end/style_guide.md#element-naming-convention)
• Removes unused qa selectors.
• Removes unused methods in page objects.
• Removes use of qa selectors in tests other than e2e. (https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/182)

Also fixes a quarantined test by updating the corresponding selector: #297257 (closed)