Draft: Update the trainee maintainer process

Why is this change being made?

As a part of WorkingGroupMaintainership, we're updating the trainee maintainership process with the goal to make it more efficent. This merge request actually removes trainee maintainer as a role and replaces it with a lighter weight process that can be applied by existing reviewers. The goal here is that we'd like to lower the burden of becoming a maintainer without sacrificing quality.

Credit to @mkozono for his suggestion in #13420 (comment 1021198892)+, I'm excited to see where we end up with this!

This change moves the burden of becoming a maintainer from the primary responsibility of an individual contributor, to a more shared responsibility with their manager.

Summary of Changes

• Trainee maintainership is removed and reviewers pursuing maintainership may instead opt to track their progress individually with their manager or mentor.
• Reviewers pursuing maintainership are encouraged to add an explaination of why they feel a merge request should be merged when handing off to a maintainer and getting feedback there.
• Generally maintainers may be submitted at any time by managers in a merge request with:
  • A brief justification of why the feel the reviewer is qualified
  • Approval from 2 existing maintainers
• Trainee maintainer mentorship program is renamed to reviewer mentorship program, but otherwise kept for folks who want to actively pursue becoming a maintainer.

View the rendered version of the changes here

Currently this is a draft, before it goes ahead, we'll also need to:

• Remove the templates for trainee maintainership and move that information to the merge request templates where needed.
• Update the maintainership merge request templates to take the new process into account.
• Update any references to the existing trainee documentation.
• Communicate this in many, many places.

CC @m_gill @nhxnguyen @mwoolf @robotmay_gitlab @sabrams @dsatcher

Relates to #13323 (closed)

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• 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
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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