Update trainee templates and references

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 efficient.

This set of merge requests 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.

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.

The original Draft MR that created this series of changes lives here: !107934 (closed) and these iterative-based MRs are inspired by this comment !107934 (comment 1038916025)

Summary of Changes

MR	You are here
The changes to the ## Maintainer section, particularly this line - it is a common misconception we've received from non-maintainers.	!109435 (merged)
The steps to take once you've become a maintainer - this is a better-defined way to accomplish what we're already doing (It should already be in the handbook)	!109436 (merged)
Asking trainees today (future reviewers) to summarize why they think something is ready to merge	!109439 (merged)
Changing the trainee->maintainer submission process from an individual advocating for themselves to manager/mentor opening the MR and advocating	!109441 (merged)
Reducing the "readiness" aspect or eligibility for maintainership from "I 'feel' ready and have many reviews in my tracking issue" to "I could be ready at any time as long as it is justified"	!109445 (merged)
Manager/mentor responsibilities process and language changes	!109448 (merged)
Update trainee template and references	🤘

View the rendered version of the changes here

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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