Revert AddIndexConcurrently to AddIndex for CI compatibility

The api-migrations CI job runs against SQLite which does not support PostgreSQL-specific AddIndexConcurrently. Standard AddIndex works across all backends and the brief write lock during production upgrades is acceptable for this table size.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

awx/main/migrations/0206_add_host_name_lower_index.py

@@ -1,17 +1,15 @@
 from django.db import migrations, models
-from django.contrib.postgres.operations import AddIndexConcurrently
 from django.db.models.functions import Lower
 
 
 class Migration(migrations.Migration):
-    atomic = False
 
     dependencies = [
         ('main', '0205_add_ordering_to_instancegroup_and_workflow_nodes'),
     ]
 
     operations = [
-        AddIndexConcurrently(
+        migrations.AddIndex(
             model_name='host',
             index=models.Index(
                 Lower('name'),