Use 'Danger' tag on background migrations section

doc/update/README.md

@@ -21,7 +21,7 @@ when updating to the current version.
 ## Background migrations
-CAUTION: **Caution:**
+DANGER: **Danger:**
 It's important to ensure that any background migrations have been fully completed
 before upgrading to a new major version. Upgrading before background migrations have
 finished may lead to data corruption.