ci: add Dependabot for GitHub Actions version updates (#1432)

Adds a `.github/dependabot.yml` configuration with a `github-actions`
ecosystem entry.

Dependabot will automatically open PRs to keep pinned SHA digests
up-to-date as new versions of Actions are released, closing the loop
opened by #1431.

---
_Generated by
[ds-security-scanning](https://github.com/developmentseed/ds-security-scanning)
zizmor-cli-unpinned-uses_

Author: lhoupert

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    groups:
+      minor-and-patch:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"