chore: change ui submodule to track 3.8 (#454)

Author: AloizioMacedo

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "src/maasui/src"]
 	path = src/maasui/src
 	url = https://github.com/canonical/maas-ui
-	branch = main
+	branch = 3.8