Set submodule update cadence to weekly

This sets the Dependabot submodule update cadence from montly to
weekly, as requested in:
#1702 (comment)

(This change in GitPython corresponds directly to
gitpython-developers/gitdb#104 in gitdb.)

Author: EliahKagan

.github/dependabot.yml

@@ -8,4 +8,4 @@ updates:
 - package-ecosystem: "gitsubmodule"
   directory: "/"
   schedule:
-    interval: "monthly"
+    interval: "weekly"