feat: switch the default k3s routing layer from traefik to nginx

The ingress-nginx ingress implementation is the standard ingress in the Kubernetes community, and it doesn't make sense for us to stray from that just to utilize the k3s default.

In the future, we might drop k3s, but this works well for now.

docs/appendices/0.34.0-migration-guide.md

@@ -3,3 +3,11 @@
 ## Removals
 
 - The `disable-chown` property of the `scheduler-docker-local` plugin has been removed. Mounted paths will no longer have file permissions changed during the pre-deploy phase. Files baked into the container image for herokuish builds will always be owned by the correct user.
+
+## Changes
+
+- The k3s scheduler now defaults to nginx as it's default proxy implementation. The traefik proxy implementation is still available, though users will need to set the global `ingress-class` k3s property to `traefik` via the following command:
+
+    ```shell
+    dokku scheduler-k3s:set --global ingress-class traefik
+    ```