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/flat-object-metadata/utils/from-create-object-input-to-flat-object-metadata-and-flat-field-metadatas-to-create.util.ts

@@ -4,8 +4,6 @@ import {
 } from 'twenty-shared/utils';
 import { v4 } from 'uuid';
 
-import { type FeatureFlagMap } from 'src/engine/core-modules/feature-flag/interfaces/feature-flag-map.interface';
-
 import { type FlatApplication } from 'src/engine/core-modules/application/types/flat-application.type';
 import { type AllFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/types/all-flat-entity-maps.type';
 import { type CreateObjectInput } from 'src/engine/metadata-modules/object-metadata/dtos/create-object.input';
@@ -20,14 +18,12 @@ type FromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCreateArgs
   {
     createObjectInput: CreateObjectInput;
     flatApplication: FlatApplication;
-    existingFeatureFlagsMap: FeatureFlagMap;
   } & Pick<AllFlatEntityMaps, 'flatObjectMetadataMaps'>;
 export const fromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCreate =
   ({
     createObjectInput: rawCreateObjectInput,
     flatApplication,
     flatObjectMetadataMaps: existingFlatObjectMetadataMaps,
-    existingFeatureFlagsMap,
   }: FromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCreateArgs): {
     flatObjectMetadataToCreate: UniversalFlatObjectMetadata & { id: string };
     relationTargetFlatFieldMetadataToCreate: UniversalFlatFieldMetadata[];
@@ -109,7 +105,6 @@ export const fromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCre
       existingFlatObjectMetadataMaps,
       sourceFlatObjectMetadata: universalFlatObjectMetadataToCreate,
       flatApplication,
-      existingFeatureFlagsMap,
     });
 
     const objectFlatFieldMetadatas: UniversalFlatFieldMetadata[] = [