Remove an old comment

We have windows stable in the CI:
https://github.com/uutils/coreutils/runs/6082164374?check_suite_focus=true
And no longer Redox:
#2550

README.md

@@ -52,8 +52,6 @@ Both can also be generated locally, the instructions for that can be found in th
 uutils follows Rust's release channels and is tested against stable, beta and nightly.
 The current oldest supported version of the Rust compiler is `1.56`.
 
-On both Windows and Redox, only the nightly version is tested currently.
-
 ## Building
 
 There are currently two methods to build the uutils binaries: either Cargo