#988 Update release notes for v6.0-M1.

• I've updated the release notes with all merged merge requests. I've also assumed !1127 (merged) and !1132 (merged) will be merged before the release on Sunday.
• For CIF, I've used a new approach, grouping the entries per tool, rather than language/deprecation/improvement. This prevents having many consecutive lines with the same prefix (the tool name). It also prevents a long list of 'improvements and fixes' that contains 80-90% of all changes. Users can also more easily read only the parts for the tools that they actually use, which applies to CIF more than the other languages as CIF has a large collection of tools for various use cases.

Addresses #988 (closed)