Changelog: Use version for headline only

[Simran-B]

Internal ref: https://github.com/arangodb/planning/issues/3098

TODO:

• Fill gaps in version history
• Add dates when versions were published
• Does it require the date like on https://github.com/arangodb/arangojs/blob/master/CHANGELOG.md so that Github creates these comparison links automatically?!

[sleto-it]

if conflicts are resolved, a compromise can be found a we can merge - thanks

[Simran-B]

@sleto-it Resolved, plus some wrapping of overly long lines

[nikita-vanyasin]

All new changelog entries are in new format now.