Increase to 8Mi the 'public-api-body-limit' setting for Rancher 2.12.x

Summary

Starting with Rancher 2.12.1 a new public-api-body-limit setting is introduced for security reason. By default it is set to 1Mi, that is too small and may block some large helm chart payloads (observed already with some CNFs).

We propose to set it to 8Mi.

(Similar to what was done with Rancher ingress with the setting of nginx.ingress.kubernetes.io/proxy-body-size: 8m)

related references

https://github.com/rancher/rancher/security/advisories/GHSA-4h45-jpvh-6p5j