Re-add [unreleased] suffix in changelog

[elenaf9]

We used to have an [unreleased] suffix in the changelog files if a version isn't released yet. It made it clear what the latest unreleased version is and easy to decide if a version bump is needed or not, without looking at crates.io.

We stopped adding this suffix, to avoid the additional work of removing it again upon release: #4894.

However, I noticed that since then we've had quite a lot of unnecessary version bumps because it's not clear anymore if the version of a crate has been bumped since the last release, resulting in fix-up PRs like #5762 #5776 #5777 #5778 #5779 #5780.

The CI checks if a version has been bumped since the last released and an CHANGELOG entry added, but it doesn't check that the version has only been bumped once.

To avoid this in the future, I think I would prefer if we go back to having the [unreleased] suffix, even if that then requires some extra work when releasing. Wdyt @jxs @dariusc93? Alternative suggestions are also welcome.

[dariusc93]

I dont mind going back to having the suffix. It could then be removed when preparing to release/publish the corresponding crate like previously done. I am also not against looking into having CI catching unpublished crates when updating changelog (though not sure if this would be desirable). I think the former would be better though since it would give noticed about unpublished versions.

[drHuangMHT]

I prefer the suffix because that gives the information directly to whoever is reading the changelog without looking through CI results. But catching unpublished crates in CI is also a nice-to-have line of defense.

[dariusc93]

Might be best to go back to using it then :)

[elenaf9]

@jxs and I had an out-of-band discussion about this. Summary is that there is a preference to not have to touch the repo when releasing, which would be needed if we had the unreleased suffix.
I am working right now on a CI check to catch an invalid version bump, see #5804.

[drHuangMHT]

Summary is that there is a preference to not have to touch the repo when releasing, we would be needed if we had the unreleased suffix.

I thought the release process is somewhat automated.