modern gnupg compatibility

### What happened?

Using commands like `helm plugin install` on a modern system with a modern GPG cause the command to error out:

helm plugin install https://github.com/...
Verifying plugin signature...
Error: plugin verification failed: open /Users/user/.gnupg/pubring.gpg: no such file or directory

This is caused by GPG moving to the new file formats used since 2.1 (> 10 years ago). See also: https://www.gnupg.org/documentation/manuals/gnupg/GPG-Configuration.html

There are several workarounds like the file conversion, but the proper fix would be for helm to support the new format or even both for better compatibility.

### What did you expect to happen?

it working.

### How can we reproduce it (as minimally and precisely as possible)?

brew install helm gnupg
helm plugin install ...

### Helm version

```console
$ helm version
version.BuildInfo{Version:"v4.1.0", GitCommit:"4553a0a96e5205595079b6757236cc6f969ed1b9", GitTreeState:"clean", GoVersion:"go1.25.6", KubeClientVersion:"v1.35"}
```

### Kubernetes version

```console
$ kubectl version
not relevant
```