Rename `Test::Instance::Airgapped` scenario to `offline environment`
As per our recommended word list, we don't recommend using airgapped
- https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#air-gap-air-gapped
offline environment
is preferred
Rename Test::Instance::Airgapped
to reflect this - Test::Instance::OfflineEnvironment
or similar?
Also update where used eg. Nightly job
For backward compatibility leave Test::Instance::Airgapped
as a pass through to the new Instance scenario.