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/DDSLoader.cpp

@@ -930,7 +930,7 @@ void DDSLoadingContext::dump_debug()
         builder.append(" DDS_ALPHA_MODE_CUSTOM"sv);
     builder.append("\n"sv);
 
-    dbgln("{}", builder.to_string());
+    dbgln("{}", builder.to_deprecated_string());
 }
 
 DDSImageDecoderPlugin::DDSImageDecoderPlugin(u8 const* data, size_t size)