[TASK] Changelog docs: Re-add v7 files into v8 & master

As proposed in
https://decisions.typo3.org/t/core-changelog-documentation-workflow-revised
the v7 Changelog files are merged back into v8 and master.

Change-Id: I83b238aa24c5cfb2b3bf5450f178b438a84f3618
Resolves: #83363
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/55127


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>