.github: Add dependabot config and thus enable dependabot (#7509)

* .github: Add dependabot config and thus enable dependabot

* Update .github/dependabot.yml

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

substrate/.github/dependabot.yml

+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    labels: ["A2-insubstantial", "B0-silent", "C1-low"]
+    schedule:
+      interval: "daily"