ci: ask Dependabot to take care of updating GitHub Actions

Every once in a while, the GitHub Actions we use (such as `checkout`,
`upload-artifact`, etc) require updates as the old versions stop
working.

Let's ask Dependabot to take care of this tedious task.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/dependabot.yml

@@ -0,0 +1,13 @@
+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+# especially
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot#enabling-dependabot-version-updates-for-actions
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions" # See documentation for possible values
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"