Check os_str_display MSRV instead of feature

This feature was stabilized, so the FormatArgs lints should check if the MSRV of
the stabilization is met, rather than checking if the feature is enabled.

tests/ui/unnecessary_path_debug_formatting.rs

@@ -25,11 +25,10 @@ fn main() {
     println!("{}", path.display());
     println!("{}", path_buf.display());
 
-    // should not fire because feature `os_str_display` is not enabled
-    println!("{:?}", os_str);
-    println!("{:?}", os_string);
-
     // positive tests
+    println!("{:?}", os_str); //~ unnecessary_debug_formatting
+    println!("{:?}", os_string); //~ unnecessary_debug_formatting
+
     println!("{:?}", path); //~ unnecessary_debug_formatting
     println!("{:?}", path_buf); //~ unnecessary_debug_formatting