fix(renovate): automerge with squash to match the repo's merge settings (#237)

Renovate automerge fails in this repo.
The shared preset uses `automergeStrategy: fast-forward`, but devxy/build-cran-binaries only allows squash merges, so Forgejo answers every merge call with `405 Method Not Allowed`.

This overrides the strategy to `squash` for this repo only, so eligible minor/patch/digest PRs merge again in the automerge window.
Same fix as devxy/k8s-talos#419.

Reviewed-on: #237

renovate.json

@@ -1,6 +1,7 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["local>devxy/renovate-config"],
+  "automergeStrategy": "squash",
   "ignorePaths": [
     "docker/**",
     ".crow/process-updates.yaml",