Drop calens in favor of conventional commits

## Preamble

There is a few things that are rather annoying about the current changelog process:

- requires a manual extra step
- needs to sync link to a PR that does not exist yet (or requires additional push)
- is not clear to contributors immediately because the check happens in PRs

## Proposed solution

Drop calens and instead use [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/). Some of us are already doing it anyway. We could then build changelog from such commits.

## Requirements

- old changelog items are persisted
- there is a commit hook checking whether the commit follows conventional commits
- new changelog items are created from the git history (we need to find a solution to set from which commit not to recreate the calens items)

## Open questions

- Do we still keep the `unreleased` section of changelog or simply build changelog only on releases?
- Should we still omit certain types of changes or print everything into the changelog? \
I get the idea behind hiding "technical" stuff from end users, but I think it makes actually sense to keep everything in the changelog here on GH. User facing changelog can be then shown e.g. on our website, blog post, etc. and there we can omit them but on GH are devs. And as a dev I would actually really want to see that in this release, some existing features are better covered by tests, some refactor happened and the app might get faster, some deps have been updated, etc.