## About the changes
- Removes the feature flag for the created_by migrations.
- Adds a configuration option in IServerOption for
`ENABLE_SCHEDULED_CREATED_BY_MIGRATION` that defaults to `false`
- the new configuration option when set on startup enables scheduling of
the two created_by migration services (features+events)
- Removes the dependency on flag provider in EventStore as it's no
longer needed
- Adds a brief description of the new configuration option in
`configuring-unleash.md`
- Sets the events created_by migration interval to 15 minutes, up from
2.
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Co-authored-by: Gastón Fournier <gaston@getunleash.io>
