TMP notifications-utils: 84.0.0 -> ???, update requirement freezing accordingly

[risicle]

This is an example app update to correspond with changes in https://github.com/alphagov/notifications-utils/pull/1155

copy_config() is now done during freeze-requirements and the results should be committed so we no longer have any bootstrapping issues. using pip-sync means we have to freeze our test requirements too as pip-sync will only install explicitly requested packages, not second-tier dependencies.