Draft a public changelog entry explaining a breaking change to users

Version 5.0 of your plugin removes the old [gallery] shortcode entirely,
replacing it with a block-based gallery. Sites still using the shortcode in
existing content will see it stop rendering after updating, unless they migrate
first.

DETAILS: [an automatic migration tool is included in 5.0 that converts existing
[gallery] shortcodes to the new block on first login after update; it's opt-in
via a one-click button in the admin notice, not automatic on update itself]

Write the public changelog entry for this release.

Requirements:
- Lead with the breaking change itself, in plain language, in the first line, not
  buried under a list of new features. Someone skimming needs to know their
  content could break before they update.
- State exactly what a user needs to do (click the migration tool in the admin
  notice) and what happens if they don't (shortcode stops rendering), not just
  "we recommend migrating."
- Recommend testing on staging before updating production, specifically because
  this is a breaking change, not boilerplate advice tacked onto every release.
- Keep new-feature announcements below the breaking-change section, don't let
  them compete for attention with something that could take down a customer's
  page.
- Link out to a migration guide if one exists; if it doesn't yet, say so and give
  a support contact instead of a dead link.

Replace the DETAILS block with your real release specifics before running this.