Docs: add note regarding upgrading an integrated K8s cluster

Problem to solve

It does not appear to be currently documented that users are able to upgrade a Kubernetes cluster that is integrated with GitLab without causing any issues.

Include the following detail as necessary:

What product or feature(s) affected?

GitLab <> Kubernetes Cluster Integration

What docs or doc section affected? Include links or paths.

• GitLab Managed Apps
• Kubernetes clusters

Proposal

We should include a section within one or both of the aforementioned docs that informs users and customers that if they need to upgrade their cluster while it is integrated with GitLab, they can do so without an issue.

Other links/references

ZD: https://gitlab.zendesk.com/agent/tickets/128775 (Internal)