Reconsider usage of the changelog action

[valadaptive]

I ran into a couple issues with the GitHub action that requires a changelog entry:

• Right after a release, the "Unreleased" section gets removed, resulting in an inevitable merge conflict between every PR that re-adds the "Unreleased" section with their own change.
• Third-party contributions in the changelog seem to link a PR number by convention, which requires you to either look through both the issues and PRs pages to intuit what the next PR number will be, or create the PR, let it fail, then amend the commit with your assigned PR number.

These seem like annoying hurdles for third-party contributions.