Use Observed Feature Flags for query modifications

Summary

In !214630 (merged), we recently introduced the ability to mark a feature flag as Observed. This means that its state is tracked in Kibana, allowing us to identify and segment changes by known feature flags.

As a precaution, we should amend our Group Archival-related changes to use this new style of feature flag management to ensure that we better understand when one of our changes causes a production issue which may otherwise be difficult to attribute.

Implementation guide

• Add the observed property to all Group Archival-related feature flags which are currently enabled
• Consider adding them to in-progress work which may not be behind a flag currently
• Monitor usage and determine when it is safe to remove the Observed state (eg. there's a limit of 10 observed flags at any one time)