Fix changelog for 2.3.6

[henrymercer]

One of the changelog entries for 2.3.6 ended up incorrectly being associated with 2.3.4 due to a bad merge conflict resolution. As a long term fix, we could store unreleased changelog entries in a separate directory and only edit CHANGELOG.md with automation.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[adityasharad]

Separately tracking unreleased entries is a good idea, though perhaps confusing for OSS contributors. Let's figure out how to make that work and minimise conflicts.

[angelapwen]

Thanks for catching. I'd noticed the same happening in the 2.3.5 release as well (a change from 2.3.5 was added to 2.3.3 as a result of a merge conflict resolution), but caught it in time during the release.