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/metadata-modules/flat-entity/constant/all-many-to-one-metadata-foreign-key.constant.ts

@@ -79,7 +79,6 @@ export const ALL_MANY_TO_ONE_METADATA_FOREIGN_KEY = {
     },
   },
   objectMetadata: {
-    dataSource: null,
     workspace: null,
     application: null,
   },

packages/twenty-server/src/engine/metadata-modules/flat-entity/constant/all-many-to-one-metadata-relations.constant.ts

@@ -139,7 +139,6 @@ export const ALL_MANY_TO_ONE_METADATA_RELATIONS = {
     },
   },
   objectMetadata: {
-    dataSource: null,
     workspace: null,
     application: null,
   },