Add a warning that this package is deprecated in favor of go-quicktest/qt

https://github.com/go-quicktest/qt is stable and uses a generic API, which has fewer footguns than the API in this package.

Moreover, it's under the go-quicktest github org, which is better in the long term.

I think the README should suggest that users consider switching over, and say that this module is not getting any further feature work.

Also, it might be useful to provide some way to rewrite existing frankban/quicktest test code to go-quicktest/quicktest. Here is the sed monstrosity I hacked together in twenty minutes, which was enough to rewrite nearly all of my test code across five repos:

```
sed -r -i \
-e 's@qt "github\.com/frankban/quicktest"@"github.com/go-quicktest/qt"@' \
-e 's@\(([tbf]), (.+), (qt\.Is[a-zA-Z]+)\b@(\1, \3(\2)@' \
-e 's@\(([tbf]), (.+), (qt\.[a-zA-Z().]+), ([^,]+)@(\1, \3(\2, \4)@' \
$(git ls-files '*.go')
go mod tidy
```