Add 8.16 release notes

[mjwolf]

Add 8.16 release notes

• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? n/a
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? n/a
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. n/a
• Have you added an entry to the CHANGELOG.next.md? n/a

[github-actions]

Documentation changes preview: https://ecs_bk_2405.docs-preview.app.elstc.co/diff