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/workspace-manager/dev-seeder/core/utils/seed-feature-flags.util.ts

@@ -50,21 +50,6 @@ export const seedFeatureFlags = async ({
         workspaceId: workspaceId,
         value: true,
       },
-      {
-        key: FeatureFlagKey.IS_ATTACHMENT_MIGRATED,
-        workspaceId: workspaceId,
-        value: true,
-      },
-      {
-        key: FeatureFlagKey.IS_NOTE_TARGET_MIGRATED,
-        workspaceId: workspaceId,
-        value: true,
-      },
-      {
-        key: FeatureFlagKey.IS_TASK_TARGET_MIGRATED,
-        workspaceId: workspaceId,
-        value: true,
-      },
       {
         key: FeatureFlagKey.IS_ROW_LEVEL_PERMISSION_PREDICATES_ENABLED,
         workspaceId: workspaceId,

packages/twenty-server/src/engine/workspace-manager/workspace-migration/constant/default-feature-flags.ts

@@ -1,7 +1,3 @@
-import { FeatureFlagKey } from 'twenty-shared/types';
+import { type FeatureFlagKey } from 'twenty-shared/types';
 
-export const DEFAULT_FEATURE_FLAGS = [
-  FeatureFlagKey.IS_ATTACHMENT_MIGRATED,
-  FeatureFlagKey.IS_NOTE_TARGET_MIGRATED,
-  FeatureFlagKey.IS_TASK_TARGET_MIGRATED,
-] as const satisfies FeatureFlagKey[];
+export const DEFAULT_FEATURE_FLAGS = [] as const satisfies FeatureFlagKey[];