GA: enable sagas by default (keep flag as escape hatch)

After sagas have baked successfully on garden and club:

1. Flip the default so sagas are enabled for all clusters
2. Ship in a release
3. Keep the feature flag in place as an escape hatch — a user who sees badness can flip back to the old path

Explicitly not in this issue: removing the flag or the old code path. That waits at least one release (see follow-up).