Remove support for classic changelog format

[felixfontein]

That format is used by Ansible 2.9 (and nothing else to my knowledge). It has been deprecated in 0.21.0 (released on 2023-06-27).

If a new relase of Ansible 2.9 should be done, an older version of antsibull-changelog still supporting that format can be used. IMO there's no need to carry around the extra complexity.

[samccann]

@felixfontein - I was poking away at this one and removing all classic support, but then I noticed this in the docs about the changes_format:

Note that support for `classic` is **DEPRECATED** and will be removed in
a future release. The field will from then on be required.

So does that mean we need to keep that parameter around even though it can only ever be combined once classic support is removed? Or can we just get rid of it entirely?

[felixfontein]

@samccann that parameter should stay around for some time, and make the program fail if set to anything else than combined. That ensures that new versions of antsibull-changelog aren't accidentally used with projects using the old format that it no longer supports.