docs: update CHANGELOG.md for v3.6.0-rc1

[github-actions]

Automated changes by create-pull-request GitHub action

[agilgur5]

@Joibel, per the releasing docs, you just need to push an empty commit to run CI, there's a sample command and message there too (that previous changelog PRs have).
The commit shouldn't contain anything, and changes to the CHANGELOG will get undone by the generator too next run.

No biggie, but just an FYI so you know

[agilgur5]

Also we should create a new 3.6 releases issue similar to #11997 and link this PR there

[Joibel]

Also we should create a new 3.6 releases issue similar to #11997 and link this PR there

This wasn't done previously until the first non-RC it seems e.g. v3.5.0

I had planned to do this when we made the release-3.6 branch. I'm not expecting this RC to be the only one because of how long it's been since 3.5, but once we've got some more testing in I may do that at RC-2 or 3, and create a tracking issue at the same time.

[agilgur5]

Originally that was because those issues were only for requests for backports, which don't apply to RCs since everything on main is included.

I still think the issue is useful purely for a place to make announcements and track all releases. Including for users to subscribe to (they can subscribe to repo releases as well, but some users are awaiting a certain feature/fix and so want more nuanced updates including a link to the CHANGELOG PR etc). That's mostly what I've used the 3.5 issue for since I've been doing backports (where I've tried to include as much as possible without specifics user requests). It's also a central place for users to upvote requests for another release and a general reference to link to for user questions