This was found not to work, and prevents releases from being made.

The cause of the issue is that the trigger-readthedocs-webhook job (in
project-config) is marked as 'final', meaning that a non-trusted Zuul
config source can't override its variables. Instead, you're supposed to
use the trigger-readthedocs-webhook project template, but only one
instance of this can be instantiated by design.

Let's revert to manually updating the release notes for now.

Change-Id: I271c972c7fdde23085f3026137806bb1e3048e5e