SearchQuery: use BigAutoField for primary key (#9671)

* SearchQuery: use BigAutoField for primary key

We have 5M records, so migration shouldn't take that long (1-2 min?),
and we use a task to create the records, so this shouldn't affect
search.

```python
In [1]: SearchQuery.objects.count()
Out[1]: 5062590
```

Ref #9492

* Linter

readthedocs/search/apps.py

@@ -2,6 +2,7 @@
 
 
 class SearchConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
     name = 'readthedocs.search'
 
     def ready(self):

readthedocs/search/migrations/0005_alter_searchquery_id.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.15 on 2022-10-18 15:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("search", "0004_make_total_results_not_null"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="searchquery",
+            name="id",
+            field=models.BigAutoField(
+                auto_created=True, primary_key=True, serialize=False, verbose_name="ID"
+            ),
+        ),
+    ]