chore: changed dependabot to less frequent updates

Signed-off-by: Manuel Zedel <manuel.zedel@northern.tech>

.github/dependabot.yml

@@ -1,14 +1,16 @@
 version: 2
 updates:
   - commit-message:
-      prefix: "chore"
+      prefix: chore
     directory: /
     package-ecosystem: gitsubmodule
     schedule:
-      interval: weekly
+      interval: monthly
+      day: saturday
   - commit-message:
-      prefix: 'chore'
+      prefix: chore
     directory: /scripts/linkbot/
     package-ecosystem: npm
     schedule:
-      interval: weekly
+      interval: monthly
+      day: saturday