Update criteria and linter for deprecations / removals

Why is this change being made?

Per discussion in Slack #release-post

Understanding:

1. Update linter in release-post-item-schema.yml to allow up to 5 deprecations / removals per file
2. Update handbook to tell author to notify the RP manager if using multiple entries in a single deprecation / removal file

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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