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/LibWebSocket/ConnectionInfo.cpp

@@ -35,7 +35,7 @@ DeprecatedString ConnectionInfo::resource_name() const
         builder.append('?');
     // the query component
     builder.append(m_url.query());
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 }