fix: Changelog format (#991)

* Fix title

* Fix changelog format

Fixes the changelog format to comply with:
- Every new changelog entry for a version is defined with an H2.
- Every sub section in a chanelog entry, such as "Fixes", is defined
with an H3.

* Reorder entries

Moves the changelog entries for versios that were misplaced (v1.16.0 to
v1.16.6) to its correct position at the top of the changelog.