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/LibGUI/FileSystemModel.h

@@ -140,7 +140,7 @@ public:
 
     static DeprecatedString timestamp_string(time_t timestamp)
     {
-        return Core::DateTime::from_timestamp(timestamp).to_string();
+        return Core::DateTime::from_timestamp(timestamp).to_deprecated_string();
     }
 
     bool should_show_dotfiles() const { return m_should_show_dotfiles; }