Clarify Senior plus maintainer projects

Why is this change being made?

We have a requirement for Senior+ engineers to be maintainers. This MR clarifies the projects this can apply to, that the team member is a contributor, and that there is a need for maintainers in that project.

This change aims to provide clarity around which projects the requirement applies to. In particular for managers working with team members looking to become senior, they need to know up front if the career development plan they are working on with their team member will meet the senior+ maintainership requirement when the team member is ready to be put forward for promotion.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
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