Fixed #31703 -- Made makemigrations name all initial migrations "initial".

When the MigrationAutodetector creates more than one initial migration
in a app, name all initial migrations "initial" rather than the opaque
"auto_<DATE>_<TIME>" name.

Initial migrations that have a descriptive name continue to use the
descriptive name.

django/db/migrations/migration.py

@@ -192,7 +192,7 @@ class Migration:
         ):
             name = '_'.join(sorted(o.migration_name_fragment for o in self.operations))
         if name is None:
-            name = 'auto_%s' % get_migration_name_timestamp()
+            name = 'initial' if self.initial else 'auto_%s' % get_migration_name_timestamp()
         return name