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/LibWeb/MimeSniff/MimeType.cpp

@@ -220,7 +220,7 @@ DeprecatedString MimeType::serialized() const
     }
 
     // 3. Return serialization.
-    return serialization.to_string();
+    return serialization.to_deprecated_string();
 }
 
 void MimeType::set_parameter(DeprecatedString const& name, DeprecatedString const& value)