Enable Dependabot again

We disabled Dependabot in commit a8d6b9b33 because it would
automatically create pull requests on every fork.

But since November 2022 Dependabot no longer behaves that way [1], so
now we can safely enable it without spamming every repository in our
community.

[1] https://github.blog/changelog/2022-11-07-dependabot-pull-requests-off-by-default-for-forks/

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+- package-ecosystem: bundler
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "04:00"
+    timezone: Etc/UTC
+  open-pull-requests-limit: 25