Curve25519 cryptography not supported in gpg decrypt

**What steps did you take:**
[A clear and concise description steps that can be used to reproduce the problem.]

Followed instructions as per https://carvel.dev/kapp-controller/docs/v0.32.0/sops/ , however, since my local gpg version is 2.3.8 (in Fedora 37 and homebrew), this defaulted to Curve25519 keys.

**What happened:**
[A small description of the issue]

Once in the cluster, sops templating fails with:

```
'Building config: Generating secring.gpg: Reading private
keys: openpgp: unsupported feature: public key type: 22'
```

**What did you expect:**
[A description of what was expected]

Decryption to succeed

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

https://github.com/golang/go/issues/44226 shows that the opengpg package has now been frozen, so this is unlikely to be ever implemented (and I more or less agree with their concerns), so maybe it's better to steer people towards age by default.

Another issue that I found was that I initially had listed the template options with

```
- ytt: {}
- sops: ...
```

and this failed with
```
usefulErrorMessage: Templating data is not supported
```

and only found to place sops first by following the function references in VSCode. A more user-facing error message may be helpful here.

**Environment:**

- kapp Controller version (execute `kubectl get deployment -n kapp-controller kapp-controller -o yaml` and the annotation is `kbld.k14s.io/images`):
- Kubernetes version (use `kubectl version`)

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