Reverting back to default merge strategy for CHANGELOG.md
As discussed in #406 (closed) and a couple of times during the quantify developers meeting, the merge strategy that was introduced in quantify-core!399 (merged) and !495 (merged), causes problems after a new release is introduced. This merge request will revert the merge strategy back to how it was before.
Motivation
The main branch and all the branches for which a merge request is opened will have conflicting changes in the file CHANGELOG.md that is not handled correctly by the union
merge strategy. During this merge, the changelog entries are moved under the released section, rather than under the unreleased section. Moreover, this often goes unnoticed.