Fix flat field metadata date type (#16445)

# Introduction
Following https://github.com/twentyhq/twenty/pull/16420

packages/twenty-server/src/engine/twenty-orm/entity-manager/workspace-entity-manager.spec.ts

@@ -130,8 +130,8 @@ describe('WorkspaceEntityManager', () => {
       objectMetadataId: 'test-entity-id',
       isNullable: true,
       isLabelSyncedWithName: false,
-      createdAt: new Date(),
-      updatedAt: new Date(),
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
       universalIdentifier: 'field-id',
       defaultValue: null,
       description: null,

packages/twenty-server/src/engine/twenty-orm/repository/__tests__/workspace.repository.spec.ts

@@ -78,8 +78,8 @@ describe('WorkspaceRepository', () => {
       icon: 'IconKey',
       workspaceId: 'test-workspace-id',
       universalIdentifier: 'id-field-universal-id',
-      createdAt: new Date(),
-      updatedAt: new Date(),
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
       defaultValue: null,
       options: null,
       settings: null,