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/engine/workspace-manager/workspace-manager.module.ts

@@ -6,7 +6,6 @@ import { FeatureFlagModule } from 'src/engine/core-modules/feature-flag/feature-
 import { UserWorkspaceEntity } from 'src/engine/core-modules/user-workspace/user-workspace.entity';
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { AiAgentModule } from 'src/engine/metadata-modules/ai/ai-agent/ai-agent.module';
-import { DataSourceModule } from 'src/engine/metadata-modules/data-source/data-source.module';
 import { FieldMetadataEntity } from 'src/engine/metadata-modules/field-metadata/field-metadata.entity';
 import { WorkspaceManyOrAllFlatEntityMapsCacheModule } from 'src/engine/metadata-modules/flat-entity/services/workspace-many-or-all-flat-entity-maps-cache.module';
 import { LogicFunctionEntity } from 'src/engine/metadata-modules/logic-function/logic-function.entity';
@@ -29,7 +28,6 @@ import { WorkspaceManagerService } from './workspace-manager.service';
     WorkspaceMigrationModule,
     ObjectMetadataModule,
     DevSeederModule,
-    DataSourceModule,
     FeatureFlagModule,
     PermissionsModule,
     AiAgentModule,