confirmation: Migration to add expiry_date - last step.

With the previous steps, all Confirmation objects should have been
migrated and the expiry_date field is being used for checking
confirmation validity - so all that's left is to set the NOT NULL
constraint on the column.

confirmation/migrations/0010_alter_confirmation_expiry_date.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.5 on 2021-08-02 19:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("confirmation", "0009_confirmation_expiry_date_backfill"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="confirmation",
+            name="expiry_date",
+            field=models.DateTimeField(db_index=True),
+        ),
+    ]