Standardize page objects use on CE Plan tests

What does this MR do?

This is the first iteration to address the following issue: https://gitlab.com/gitlab-org/quality/team-tasks/issues/168.

I intend to document the standard in another MR.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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