Autogenerate changelogs by using PR templates

[burmanm]

What is missing?

Currently CHANGELOG changes will cause a lot of merge conflicts and unnecessary pushes to update them. Since we have to fill the PR template already, we could add the logic there. Simply add a new field "CHANGELOG entry" which would then get parsed when we create a new release of cass-operator.

Why is this needed?

Merge conflicts are annoying and require rebases. Also, there's sometimes need to make additional changes to the changelog after the merge has already happened. And we in any case have to detail the changes in the PR template.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: CASS-5