Remove Solaris from unsupported list

We now support it via the poll(2) implementation.

It's also `cargo check`ed in our CI already.

README.md

@@ -152,10 +152,8 @@ This uses the Windows AFD system to access socket readiness events.
 
 ### Unsupported
 
-* Solaris, see [issue #1152]
 * Wine, see [issue #1444]
 
-[issue #1152]: https://github.com/tokio-rs/mio/issues/1152
 [issue #1444]: https://github.com/tokio-rs/mio/issues/1444
 
 ## MSRV Policy