Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibGfx/PortableImageLoaderCommon.h

@@ -45,7 +45,7 @@ static bool read_number(Streamer& streamer, TValue* value)
         sb.append(byte);
     }
 
-    auto const opt_value = sb.to_string().to_uint();
+    auto const opt_value = sb.to_deprecated_string().to_uint();
     if (!opt_value.has_value()) {
         *value = 0;
         return false;