[11.x] Fix retrieving generated columns on legacy PostgreSQL (#50834)

* fix retrieving generated column on pgsql < 12

* flip the condition

src/Illuminate/Database/Schema/Grammars/PostgresGrammar.php

@@ -123,7 +123,7 @@ public function compileColumns($schema, $table)
             .'(select tc.collcollate from pg_catalog.pg_collation tc where tc.oid = a.attcollation) as collation, '
             .'not a.attnotnull as nullable, '
             .'(select pg_get_expr(adbin, adrelid) from pg_attrdef where c.oid = pg_attrdef.adrelid and pg_attrdef.adnum = a.attnum) as default, '
-            .'a.attgenerated as generated, '
+            .(version_compare($this->connection?->getServerVersion(), '12.0', '<') ? "'' as generated, " : 'a.attgenerated as generated, ')
             .'col_description(c.oid, a.attnum) as comment '
             .'from pg_attribute a, pg_class c, pg_type t, pg_namespace n '
             .'where c.relname = %s and n.nspname = %s and a.attnum > 0 and a.attrelid = c.oid and a.atttypid = t.oid and n.oid = c.relnamespace '