change severity of G-3120 to blocker

because a missing alias in a subquery can cause an
unexpected behaviour, a bug.

Author: PhilippSalvisberg

docs/4-language-usage/3-dml-and-sql/1-general/g-3120.md

@@ -1,6 +1,6 @@
 # G-3120: Always use table aliases when your SQL statement involves more than one source.
 
-!!! warning "Major"
+!!! bug "Blocker"
     Maintainability
 
 ## Reason