Ensure migration version is 7.0

Now that Rails main is going to be Rails 7.0 instead of 6.2 we need to
make sure the migration version matches.

Followup to 1b455e2e9d

activerecord/lib/active_record/migration/compatibility.rb

@@ -13,9 +13,9 @@ module ActiveRecord
         const_get(name)
       end
 
-      V6_2 = Current
+      V7_0 = Current
 
-      class V6_1 < V6_2
+      class V6_1 < V7_0
       end
 
       class V6_0 < V6_1