Update CHANGELOG for 8.8.1

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CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
-v 8.8.1 (unreleased)
+v 8.8.1
+  - Add documentation for the "Health Check" feature
+  - Allow anonymous users to access a public project's pipelines
   - Fix MySQL compatibility in zero downtime migrations helpers
   - Fix the CI login to Container Registry (the gitlab-ci-token user)
   - Fix access to Pipelines by Anonymous user
-v 8.8.0 (unreleased)
+v 8.8.0
   - Implement GFM references for milestones (Alejandro Rodríguez)
   - Snippets tab under user profile. !4001 (Long Nguyen)
   - Fix error when using link to uploads in global snippets