CHANGELOG: add missing entry about 3.12 requiring `read:org` scope

Created by: slimsag

A customer noticed read:org permissions were now required and it was not explained at all in our changelog why this change was made. I was only able to find out why by locating our requested permissions code, backtracking that to a PR https://github.com/sourcegraph/sourcegraph/pull/7555 and then backtracking that to the actual feature that requires these permissions.

Any time we ask for new permissions please document it clearly in the CHANGELOG otherwise it will not be clear to users what we're doing and why -- something that I feel is especially important when altering permissions we request from code-hosts. i.e. simply documenting the feature is not enough, we must also document why we're changing the required permissions / scopes.