Add migration for CustomLink button_class default

netbox/extras/migrations/0060_customlink_button_class.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('extras', '0059_exporttemplate_as_attachment'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='customlink',
+            name='button_class',
+            field=models.CharField(default='outline-dark', max_length=30),
+        ),
+    ]