chore: downgrade conventional-changelog and remove duplicate entries

This MR fixes our automated changelog generation and removes the duplicate CHANGELOG entries.

We updated conventional-changelog-cli to version 3 !830 (merged)

Since then every new release produced full duplicated changelog (each release calls the version script conventional-changelog -p angular -i CHANGELOG.md -s ).

I spent couple of hours looking for the exact root cause and then I gave up and created a GitHub issue: https://github.com/conventional-changelog/conventional-changelog/issues/1062