Experiments using Flipper will require Feature Flag YML files

Problem

Soon all development feature flags will be required to have a YML file, including experiments.

See #276170 (comment 459330785)

Next steps

• gitlab-experiment introduces the :experiment feature flag type in !45840 (merged)
• experimentation.rb experiments will also require a YML file !50909 (merged)

cc @gitlab-org/growth/experiment-devs