Update steps for announcing deprecations in release post

Why is this change being made?

Adds steps to edit existing deprecations and clarifies the change from 14.5 that deprecations are only linked from the release post.

Addresses this item from 14.5 Release Post Retro: #12643 (comment 733713010)

Updated sections:

Before	After
Monthly releases	Monthly releases
(TW reviewers) Update the deprecations doc	(TW reviewers) Update the deprecations doc
Adding, editing, or removing merged content blocks after the 18th and before the 22nd	Adding, editing, or removing merged content blocks after the 18th and before the 22nd
Adding, editing, or removing merged content blocks after the 22nd	Adding, editing, or removing merged content blocks after the 22nd
Deprecations, removals and breaking changes	Deprecations, removals and breaking changes
Deprecations	Deprecations
Creating a deprecation entry	Creating a deprecation entry
(New section)	Editing a deprecation entry

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in 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.

---