Add renovate config (#408)

+ write access required for `renovate-bot` user

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/408
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

renovate.json5

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "local>gitea/renovate-config"
+  ]
+}