github: add dependabot

It will run weekly and open a PR whenever any of the actions are out of
date. Minor change is required in the repo settings - see the docs link

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.github/dependabot.yml

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