## Summary
- Remove 3 completed migration feature flags: `IS_ATTACHMENT_MIGRATED`,
`IS_NOTE_TARGET_MIGRATED`, `IS_TASK_TARGET_MIGRATED` — these were
already enabled by default for all new workspaces via
`DEFAULT_FEATURE_FLAGS`
- Delete all upgrade command directories for versions <= 1.18 (`1-16/`,
`1-17/`, `1-18/`) along with their module registrations, removing ~6,600
lines of dead migration code
- Simplify frontend utility functions
(`getActivityTargetObjectFieldIdName`, `getActivityTargetsFilter`,
`getActivityTargetFieldNameForObject`,
`generateActivityTargetMorphFieldKeys`,
`findActivitiesOperationSignatureFactory`) by removing the
`isMorphRelation` parameter and always using the morph relation path
- Remove feature flag checks from 7 frontend hooks/components that were
gating attachment and activity target behavior behind the removed flags
- Simplify `buildDefaultRelationFlatFieldMetadatasForCustomObject`
server util to always treat attachment, noteTarget, and taskTarget as
morph relations without checking feature flags
