fix(migrations): Mark group message migration as dangerous (#78983)

This migration took down sentry due to our statement timeouts failing.
Marking it dangerous for now so that we can resume deploys.

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src/sentry/migrations/0776_drop_group_score_in_database.py

@@ -18,7 +18,7 @@ class Migration(CheckedMigration):
     #   is a schema change, it's completely safe to run the operation after the code has deployed.
     # Once deployed, run these manually via: https://develop.sentry.dev/database-migrations/#migration-deployment
 
-    is_post_deployment = False
+    is_post_deployment = True
 
     dependencies = [
         ("sentry", "0775_add_dashboard_permissions_model"),