refactor: drop setMigrationMode from MigrationBackfillStorage

migration_mode is already a @JsonProperty on PostgreSQLConfig (and exposed
via getValidFields()), so the existing multitenancy CRUD endpoint already
covers updating it via coreConfig:

  PUT /recipe/multitenancy/connectionuridomain/v2
  { "coreConfig": { "migration_mode": "DUAL_WRITE_READ_OLD" } }

That path persists via overwriteTenantConfig + refreshAfterKnownTenantChange,
which rebuilds the storage's config from the persisted JSON. No separate
storage-interface write method is needed.

Author: porcellus

src/main/java/io/supertokens/pluginInterface/migration/MigrationBackfillStorage.java

@@ -34,14 +34,15 @@ public interface MigrationBackfillStorage extends Storage {
 
     /**
      * Returns the current migration mode from the storage's configuration.
+     *
+     * To change the migration mode at runtime, update the storage's core_config via the
+     * existing multitenancy CRUD endpoint (e.g. PUT /recipe/multitenancy/connectionuridomain/v2
+     * with { "coreConfig": { "migration_mode": "..." } }). That path persists through
+     * overwriteTenantConfig and triggers a config refresh, after which this method returns
+     * the new value.
      */
     MigrationMode getMigrationMode();
 
-    /**
-     * Persists a new migration mode for this storage (CUD).
-     */
-    void setMigrationMode(MigrationMode mode) throws StorageQueryException;
-
     /**
      * Returns the count of users that still need backfilling.
      * A user needs backfilling when their time_joined is 0 in app_id_to_user_id,