databases/mongodb70: add new port

Mind that this is still a RC.

Changelog:
https://www.mongodb.com/docs/v7.0/release-notes/7.0/

* based on databases/mongodb60
* disabled --use-system-icu as version from ports gives an error on start-up:
  https://www.mongodb.com/community/forums/t/mongodb-7-0-rc0-cycle-in-dependency-graph-loadicudata/224571
* patch to compile asio-master with c++20
* commented out 'metrics_client' in SConstruct, symbol is not available in the repo, doesn't seem to be needed for the build