Why change v8 to spidermonkey in mongodb js engine?

i am learning your mongodb source code. i think spidermonkey is not so very meaningful. i don’t want to learn another js engine. i want to contribute to you in the future.
although i can not master C++ 100%, but if i master rust, i would master c++ 100%. it wll cost me so long time. do you know?
by the way, based on your docs, your mongodb source code is written by c++ 11 syntax largely. c++ 20 is the stable release now. oh my god. your syntax is so old, and if written by latest c++ syntax, your source code can be decreased very much.