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
+    ```

docs/deployment/schedulers/k3s.md

@@ -58,12 +58,12 @@ By default, Dokku will attempt to auto-detect the IP address of the server. In c
 dokku scheduler-k3s:initialize --server-ip 192.168.20.15
 ```
 
-Dokku's k3s integration natively uses `Traefik` as it's ingress load balancer via [Traefik's CRDs](https://doc.traefik.io/traefik/providers/kubernetes-crd/), but a cluster can be set to use `nginx` via the [ingress-nginx](https://github.com/kubernetes/ingress-nginx) project. Switching to nginx will result in any `nginx` plugin settings being respected, either by turning them into annotations or creating a custom server snippet.
+Dokku's k3s integration natively uses `nginx` as it's ingress load balancer via [ingress-nginx](https://github.com/kubernetes/ingress-nginx). Properties set by the `nginx` plugin will be respected, either by turning them into annotations or creating a custom server/location snippet that the `ingress-nginx` project can use.
 
-To change the ingress, set the `--ingress-class` flag:
+Dokku can also use Traefik on cluster initialization via the [Traefik's CRDs](https://doc.traefik.io/traefik/providers/kubernetes-crd/). To change the ingress, set the `--ingress-class` flag:
 
 ```shell
-dokku scheduler-k3s:initialize --ingress-class nginx
+dokku scheduler-k3s:initialize --ingress-class traefik
 ```
 
 ### Adding nodes to the cluster

plugins/scheduler-k3s/scheduler_k3s.go

@@ -42,7 +42,7 @@ var (
 	}
 )
 
-const DefaultIngressClass = "traefik"
+const DefaultIngressClass = "nginx"
 const GlobalProcessType = "--global"
 const KubeConfigPath = "/etc/rancher/k3s/k3s.yaml"
 const DefaultKubeContext = ""