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-front/src/modules/object-record/record-field/ui/meta-types/input/components/RichTextFieldEditor.tsx

@@ -23,7 +23,6 @@ import { usePushFocusItemToFocusStack } from '@/ui/utilities/focus/hooks/usePush
 import { useRemoveFocusItemFromFocusStackById } from '@/ui/utilities/focus/hooks/useRemoveFocusItemFromFocusStackById';
 import { FocusComponentType } from '@/ui/utilities/focus/types/FocusComponentType';
 import { useHotkeysOnFocusedElement } from '@/ui/utilities/hotkey/hooks/useHotkeysOnFocusedElement';
-import { useIsFeatureEnabled } from '@/workspace/hooks/useIsFeatureEnabled';
 import { t } from '@lingui/core/macro';
 import '@blocknote/core/fonts/inter.css';
 import '@blocknote/mantine/style.css';
@@ -32,7 +31,6 @@ import '@blocknote/react/style.css';
 import { Key } from 'ts-key-enum';
 import { isDefined } from 'twenty-shared/utils';
 import { useDebouncedCallback } from 'use-debounce';
-import { FeatureFlagKey } from '~/generated-metadata/graphql';
 
 type RichTextFieldEditorProps = {
   recordId: string;
@@ -82,13 +80,8 @@ export const RichTextFieldEditor = ({
 
   const focusId = `${recordId}-${fieldName}`;
 
-  const isAttachmentMigrated = useIsFeatureEnabled(
-    FeatureFlagKey.IS_ATTACHMENT_MIGRATED,
-  );
-
   const attachmentTargetFieldIdName = getActivityTargetObjectFieldIdName({
     nameSingular: objectNameSingular,
-    isMorphRelation: isAttachmentMigrated,
   });
 
   const { records: attachments } = useFindManyRecords<Attachment>({