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/metadata-modules/object-metadata/object-metadata.service.ts

@@ -406,7 +406,6 @@ export class ObjectMetadataService extends TypeOrmQueryService<ObjectMetadataEnt
       createObjectInput,
       flatApplication: resolvedOwnerFlatApplication,
       flatObjectMetadataMaps: existingFlatObjectMetadataMaps,
-      existingFeatureFlagsMap,
     });
 
     const flatDefaultViewToCreate = this.computeFlatViewToCreate({

packages/twenty-server/src/engine/metadata-modules/object-metadata/utils/build-default-relation-flat-field-metadatas-for-custom-object.util.ts

@@ -2,10 +2,9 @@ import {
   STANDARD_OBJECTS,
   DEFAULT_RELATIONS_OBJECTS_STANDARD_IDS,
 } from 'twenty-shared/metadata';
-import { FieldMetadataType, FeatureFlagKey } from 'twenty-shared/types';
+import { FieldMetadataType } from 'twenty-shared/types';
 import { capitalize, isDefined } from 'twenty-shared/utils';
 
-import { type FeatureFlagMap } from 'src/engine/core-modules/feature-flag/interfaces/feature-flag-map.interface';
 import { RelationType } from 'src/engine/metadata-modules/field-metadata/interfaces/relation-type.interface';
 
 import { type FlatApplication } from 'src/engine/core-modules/application/types/flat-application.type';
@@ -34,14 +33,7 @@ const morphIdByRelationObjectNameSingular = {
   string | null
 >;
 
-// TODO: once we have finished migrating, we can delete custom code
-// once we migrate timeline activity to morph relations, we can add it.
-// another way to check if an object is migrated to morph relations is to check if the feature flag is enabled
-const DEFAULT_MORPH_RELATIONS_OBJECTS_STANDARD_IDS =
-  [] as const satisfies (keyof typeof STANDARD_OBJECTS)[];
-
 export type BuildDefaultRelationFieldsForCustomObjectArgs = {
-  existingFeatureFlagsMap: FeatureFlagMap;
   existingFlatObjectMetadataMaps: FlatEntityMaps<FlatObjectMetadata>;
   sourceFlatObjectMetadata: UniversalFlatObjectMetadata;
   flatApplication: FlatApplication;
@@ -58,7 +50,6 @@ const EMPTY_SOURCE_AND_TARGET_FLAT_FIELD_METADATAS_RECORD: SourceAndTargetFlatFi
   };
 
 export const buildDefaultRelationFlatFieldMetadatasForCustomObject = ({
-  existingFeatureFlagsMap,
   existingFlatObjectMetadataMaps,
   sourceFlatObjectMetadata,
   flatApplication,
@@ -79,21 +70,11 @@ export const buildDefaultRelationFlatFieldMetadatasForCustomObject = ({
   const result =
     DEFAULT_RELATIONS_OBJECTS_STANDARD_IDS.reduce<SourceAndTargetFlatFieldMetadatasRecord>(
       (sourceAndTargetFlatFieldMetadatasRecord, objectMetadataNameSingular) => {
-        const isObjectMigratedFromOlderReleases =
-          DEFAULT_MORPH_RELATIONS_OBJECTS_STANDARD_IDS.map(toString).includes(
-            objectMetadataNameSingular,
-          );
-        const isFeatureFlagEnabled =
-          objectMetadataNameSingular === 'timelineActivity' ||
-          (objectMetadataNameSingular === 'attachment' &&
-            existingFeatureFlagsMap[FeatureFlagKey.IS_ATTACHMENT_MIGRATED]) ||
-          (objectMetadataNameSingular === 'noteTarget' &&
-            existingFeatureFlagsMap[FeatureFlagKey.IS_NOTE_TARGET_MIGRATED]) ||
-          (objectMetadataNameSingular === 'taskTarget' &&
-            existingFeatureFlagsMap[FeatureFlagKey.IS_TASK_TARGET_MIGRATED]) ||
-          false;
         const isObjectMigratedToMorphRelations =
-          isObjectMigratedFromOlderReleases || isFeatureFlagEnabled;
+          objectMetadataNameSingular === 'timelineActivity' ||
+          objectMetadataNameSingular === 'attachment' ||
+          objectMetadataNameSingular === 'noteTarget' ||
+          objectMetadataNameSingular === 'taskTarget';
 
         const targetFlatObjectMetadataId =
           objectIdByNameSingular[objectMetadataNameSingular];