chore: remove completed migration feature flags and upgrade commands <= 1.18 (#19074)

## 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

packages/twenty-server/src/engine/twenty-orm/entity-manager/workspace-entity-manager.spec.ts

@@ -239,9 +239,6 @@ describe('WorkspaceEntityManager', () => {
         IS_PUBLIC_DOMAIN_ENABLED: false,
         IS_EMAILING_DOMAIN_ENABLED: false,
         IS_DASHBOARD_V2_ENABLED: false,
-        IS_ATTACHMENT_MIGRATED: false,
-        IS_NOTE_TARGET_MIGRATED: false,
-        IS_TASK_TARGET_MIGRATED: false,
         IS_ROW_LEVEL_PERMISSION_PREDICATES_ENABLED: false,
         IS_JUNCTION_RELATIONS_ENABLED: false,
         IS_COMMAND_MENU_ITEM_ENABLED: false,