#1650)

For example:

```swift
@test<[Int]>(arguments: [1, 2, 3]) func f(i: Int) { ... }
```

We never intended for this to be valid syntax. It appears some folks
_are_ using it, so deprecate for now and make it an error in the next
language mode. The diagnostic looks like this in the Swift 6 language
mode:

> ⚠️ Generic argument clause of attribute 'Test' is unsupported; this is
an error in the Swift 7 language mode

And in the Swift 7 language mode, it's an error. (In the future, if we
find that this syntax is useful and want to build out some sort of
support for it, we can of course remove the diagnostic.)

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.