[OBJECT_CACHE_FLUSH_REQUIRED_WHEN_RELEASED] Remove FlatObjectMetadata custom fieldMetadataIds fk aggregator property (#17438)

# Introduction
Currently refactoring `flatEntity` typing, encountering some tsc errors
due to this fk aggregator custom override
It shall now follow generic pattern leading to be named `fieldIds`

Needs to flush object cache when released

packages/twenty-server/src/engine/metadata-modules/flat-object-metadata/__mocks__/get-flat-object-metadata.mock.ts

@@ -14,7 +14,7 @@ export const getFlatObjectMetadataMock = (
   return {
     viewIds: [],
     indexMetadataIds: [],
-    fieldMetadataIds: [],
+    fieldIds: [],
     description: 'default flat object metadata description',
     icon: 'icon',
     id: faker.string.uuid(),