linux-rt_latest: 5.9.1-rt20 -> 5.10-rt17

This does not keep 5.9 series because it has known issues and rt
upstream has stopped maintaining it in favor of 5.10.

"ls -v" sorts by version, making 5.10 the next $prev kernel.

pkgs/os-specific/linux/kernel/update-rt.sh

@@ -58,7 +58,7 @@ update-if-needed() {
         msg="$nixattr: $cur -> $new"
     else
         msg="$nixattr: init at $new"
-        prev=$(ls "$(dirname "$0")"/linux-rt-*.nix | tail -1)
+        prev=$(ls -v "$(dirname "$0")"/linux-rt-*.nix | tail -1)
         cp "$prev" "$file"
         cur=$(file-version "$file")
     fi