Setting the OpenAPI spec version not working

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

I'm trying to set the OpenAPI spec version to 3.0 for the time being, yet neither the delegate method nor the MSBuild method proposed in the docs are working.

I need it as 3.0 for the correct OpenAPI.json file to be generated, and have tried this:

**Program.cs:**
```cs
builder.Services.AddOpenApi(options =>
{
options.OpenApiVersion = OpenApiSpecVersion.OpenApi3_0;
});
```

**BackendService.csproj:**
```cs

.



--openapi-version OpenApi3_0


```

Yet the generated OpenAPI.json is always version 3.1:
OpenAPI.json:
```
"openapi": "3.1.1",
"info": {
"title": "BackendService | v1",
"version": "1.0.0"
}
```

Any suggestions gladly appreciated!

### Expected Behavior

_No response_

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version

.NET 10

### Anything else?

_No response_