CI: Add dependabot.yml

This will cause dependabot to open a PR when various actions are
updated, provided that the action maintainer has issued a release.

Closes #12623

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.reuse/dep5

@@ -96,3 +96,7 @@ License: curl
 Files: .mailmap
 Copyright: Daniel Stenberg, <daniel@haxx.se>, et al.
 License: curl
+
+Files: .github/dependabot.yml
+Copyright: Daniel Stenberg, <daniel@haxx.se>, et al.
+License: curl