If a positional argument is provided, the use_ipv6 keyword
option is ignored:

```ruby
Resolv.new([Resolv::DNS.new], use_ipv6: false)
```

Issue a warning in this case.

Currently, you have to pass a positional hash argument to set
the DNS config.  However, after support for the use_ipv6 keyword
argument is removed, you will be able to pass either a positional
hash or keyword arguments.  Code that was just specifying the
use_ipv6 keyword optional will still work correctly in that case.

https://github.com/ruby/resolv/commit/5ff9dda991