Update qa selectors for MR page objects

What does this MR do?

• Removes deprecated qa classes for e2e merge request page objects 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 use of qa selectors in tests other than e2e. (https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/182)