kubectl diff requires update / patch permission

**What happened**:

Usually, when we think about using `diff` to compare two things - write access is not required. In this case, it seems `kubectl diff` requires write access to compare the current and future state. This might be an upstream problem if this is due to the server side apply.

When doing `kubectl diff` on a Deployment:
```
Error from server (Forbidden): deployments.extensions "my-super-deployment" is forbidden:
User "myuser" cannot patch resource "deployments" in API group "extensions" in the namespace
"mynamespace": requires one of ["container.deployments.update"] permission(s).
```

It seems to require PATCH / PUT permission under `/apis/apps/v1/namespaces/{namespace}/deployments/*` or older `/apis/extensions/v1beta1/namespaces/{namespace}/deployments/*`.

**What you expected to happen**:

Diff output to be shown.

**How to reproduce it (as minimally and precisely as possible)**:

Attempt to diff a Deployment resource from a user lacking write access to the deployment. The permission in the example above is mapped to GKE specifically.

**Anything else we need to know?**:

**Environment**:
- Kubernetes client and server versions (use `kubectl version`):
```
Client Version: version.Info{Major:"1", Minor:"19", GitVersion:"v1.19.3", GitCommit:"1e11e4a2108024935ecfcb2912226cedeafd99df", GitTreeState:"clean", BuildDate:"2020-10-14T12:50:19Z", GoVersion:"go1.15.2", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"15+", GitVersion:"v1.15.12-gke.20", GitCommit:"0ac5f81eecab42bff5ef74f18b99d8896ba7b89b", GitTreeState:"clean", BuildDate:"2020-09-09T00:48:20Z", GoVersion:"go1.12.17b4", Compiler:"gc", Platform:"linux/amd64"}
```
- Cloud provider or hardware configuration: Google, GKE
- OS (e.g: `cat /etc/os-release`): n/a

Start at the kubectl diff command and trace its requests to the Deployment API paths listed in the report, comparing the observed PATCH/PUT permission requirement with the expected read-only behavior. Reproduce with a user lacking write access and verify that the resulting diff is shown without update or patch permission.

Written by the indexing model from the issue text.