Reduce the number of dependabot C++ checks (#1939)

* Only have dependabot check monthly

Signed-off-by: Darby Johnston <darbyjohnston@yahoo.com>

Author: darbyjohnston

.github/dependabot.yml

@@ -15,5 +15,4 @@ updates:
   - package-ecosystem: "gitsubmodule"
     directory: "/"
     schedule:
-      day: "monday"
-      interval: "weekly"
+      interval: "quarterly"