Userland: Replace most printf-style APIs with AK::Format APIs :^)

Author: https://github.com/linusg
Commit: f5c35fccca
Pull-request: https://github.com/SerenityOS/serenity/pull/7632
Reviewed-by: https://github.com/awesomekling

Userland/Utilities/md.cpp

@@ -51,7 +51,7 @@ int main(int argc, char* argv[])
         success = file->open(Core::OpenMode::ReadOnly);
     }
     if (!success) {
-        fprintf(stderr, "Error: %s\n", file->error_string());
+        warnln("Error: {}", file->error_string());
         return 1;
     }
 
@@ -67,10 +67,10 @@ int main(int argc, char* argv[])
     auto document = Markdown::Document::parse(input);
 
     if (!document) {
-        fprintf(stderr, "Error parsing\n");
+        warnln("Error parsing");
         return 1;
     }
 
     String res = html ? document->render_to_html() : document->render_for_terminal(view_width);
-    printf("%s", res.characters());
+    out("{}", res);
 }