security_opt allows value without a dash

[`security_opt`](https://github.com/compose-spec/compose-spec/blob/main/spec.md#security_opt) element should accept a list (array?):
```yml
security_opt:
- label:user:USER
- label:role:ROLE
```
At least for a single value, the dash is not enforced here, so following syntax is possible for podman-compose:
```yml
security_opt:
label=disable
```
Docker-compose doesn't allow that:
```console
$ docker compose up
validating docker-compose.yml: services..security_opt must be a array
```
Following syntax works there:
```yml
security_opt:
- label=disable
```
```console
$ podman-compose version
podman-compose version 1.5.0
podman version 5.8.2
```
**Environment:**
- OS: Linux

Start by locating the security_opt validation or parsing entry point in podman-compose and compare its handling of a scalar value with the array syntax shown in the issue. Done means a scalar such as label=disable is rejected while the list form is accepted, with regression coverage for both cases.

Written by the indexing model from the issue text.