Remove DataSourceService and clean up datasource migration logic (#19532)

## Summary

- **Drop the `objectMetadata.dataSourceId` foreign key and index** via a
1-22 fast instance command — column kept nullable for data preservation
- **Delete `DataSourceService`, `DataSourceModule`, and
`DataSourceException`** — all code now uses `workspace.databaseSchema`
directly
- **Remove `IS_DATASOURCE_MIGRATED` feature flag** from default flags
and all branching logic
- **Simplify workspace/object creation pipelines** —
`WorkspaceManagerService`, `DevSeederService`, and the object creation
action handler no longer route through `DataSourceService`
- **Keep `DataSourceEntity` and the `dataSource` table** for historical
data — entity stripped of all ORM relations

packages/twenty-server/src/utils/__test__/get-object-metadata-entity.mock.ts

@@ -2,7 +2,6 @@ import { faker } from '@faker-js/faker';
 
 import { type ApplicationEntity } from 'src/engine/core-modules/application/application.entity';
 import { type WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
-import { type DataSourceEntity } from 'src/engine/metadata-modules/data-source/data-source.entity';
 import { type ObjectMetadataEntity } from 'src/engine/metadata-modules/object-metadata/object-metadata.entity';
 
 export type GetMockObjectMetadataEntityOverride =
@@ -21,7 +20,6 @@ export const getMockObjectMetadataEntity = (
     workspace: {} as WorkspaceEntity,
     createdAt: new Date(),
     updatedAt: new Date(),
-    dataSource: {} as DataSourceEntity,
     dataSourceId: faker.string.uuid(),
     description: 'default object metadata description',
     duplicateCriteria: [],