Update Dependabot configuration

This commit updates the timing of the Dependabot checks. It also updates
Dependabot to check for version changes to GitHub Actions.

Ref:
- https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#configuration-options-for-the-dependabotyml-file
- https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem

.github/dependabot.yml

@@ -0,0 +1,16 @@
+---
+version: 2
+
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: weekly
+      time: "02:00"
+      timezone: "Etc/UTC"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: weekly
+      time: "02:00"
+      timezone: "Etc/UTC"