Add a step to close any trainee maintainer issues during maintainership MR approval

Why is this change being made?

It was raised in !117931 (comment 1243378530) that it can be confusing for maintainers looking to approve engineers for maintainership when their trainee maintainer issue remains opened. Thanks to @DylanGriffith for surfacing this, I think it's worth updating our templates to make it explicit that the manager closes out any trainee maintainer issue as that's no longer part of our maintainership approval process. We can phase this step out over time - e.g. revisit this template and remove this step in 1-2 quarters.

Assigning to Maintainership WG DRIs for review 👋

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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