profiles: explicitely set prompt configuration for do-release-upgrade
After https://github.com/canonical/auto-upgrade-testing/pull/12,
setting do_release_upgrade_prompt is supported for explicitely choosing
which upgrade path to take. This makes sure the `do-release-upgrade`
configuration option `prompt` is at the correct value for existing
profiles.
profiles: add b->n and f->n upgrade for desktop and server
Following https://github.com/canonical/auto-upgrade-testing/pull/11, we
can start testing multiple LTS upgrades, so let's do it. Only desktop
and server are needed at this point, as testing every flavor will
quickly add far too much load for the infra.