Docs for SERVER-28524: remove all support for group command

Description:

Must mark that the group command has been entirely deleted. Users should use the aggregation framework instead.

Scope of changes:

• 4.2.txt
• 4.2-compat
• rm group cmd and db.collection.group method and associated files
  • apiargs
• rm mentions of group/db.collection.group

Impact to other docs outside of this product:

MVP:

Resources:

Engineering Ticket Description:

group command was deprecated in 3.4.0 (SERVER-14193).

This ticket is to track the removal of all support for JS group command in the server.