Prepare changelog for v8.16.0

[mjwolf]

Prepare changelog for 8.16.0 release

• 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] (https://github.com/elastic/ecs/blob/main/CHANGELOG.next.md)? n/a

[taylor-swanson]

The 8.16.0 (Feature Freeze) section should live in CHANGELOG.next.md at this stage, looking at the previous release's PR.

[mjwolf]

The 8.16.0 (Feature Freeze) section should live in CHANGELOG.next.md at this stage, looking at the previous release's PR.

I've moved it back for now