Setup Dependabot (#675)

Updating manually, like in #426, #641, requires checking for
dependency updates every time. Automating this with Dependabot will
save us a lot of work.

If we set open-pull-requests-limit to 0, we can only get security
updates. Dependabot will check updates every Monday if
`schedule.interval` is set to weekly.

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"