We were using the Draft4Validator in one place, so this updates it to
the Draft7Validator instead.
The schemas were mostly using draft 4 of the JSON schema, though there
were a couple of schemas that were already of version 7. This updates
them all to version 7, which is the latest version fully supported by
the jsonschema Python package. There are some breaking changes in the
newer version of the schema, but I could not see anywhere would these
affect us. Some of these schemas were not valid in version 4, but are
now valid in version 7 because `"required": []` was not valid in earlier
versions.
This will allow admin to pass through a value of "government" for the
broadcast_channel. We don't have any logic around the value of service.broadcast_channel,
so no updates are needed to the tasks etc.
We want to replace the value `None` for
service.allowed_broadcast_provider with the value of "all". As a first
step, we need to allow both values. Once notifications-admin has been
changed to pass through "all" and all the data in the database has been
updated, we can update the code to stop supporting both values.
