Change dependabot to PR against dependencies branch fixes #2944 (#2945)

.github/dependabot.yml

@@ -5,13 +5,16 @@ updates:
   schedule:
     interval: weekly
   open-pull-requests-limit: 99
+  target_branch: dependencies
 - package-ecosystem: npm
   directory: "/app/experimenter/static/core"
   schedule:
     interval: weekly
   open-pull-requests-limit: 99
+  target_branch: dependencies
 - package-ecosystem: pip
   directory: "/app/requirements"
   schedule:
     interval: weekly
   open-pull-requests-limit: 99
+  target_branch: dependencies