The sharded cluster upgrade procedure will change for 8.0.

In the current procedure, the binaries are upgraded in a certain order (config server, shards, routers), then the FCV is upgraded. The new procedure is described in WRITING-15673, and can be summarized as follows: node binaries can now be upgraded in any order, but when the new binary is brought up, it must receive the ` --upgradeBackCompat` parameter in the command line. Once the whole cluster binaries are upgraded, they are all restarted, removing the `--upgradeBackCompat` parameter, again in any order. After all binaries have been restarted, the FCV can be upgraded just as before.

Each node replica set can be upgraded internally in the same order as before (first secondaries, then the primary after stepping down). The restart can also follow the same order.

Description of Linked Ticket

This ticket is about introducing the --upgradeBackCompat command line parameter to both MongoD and MongoS along with the respective change in wire protocol version behaviour as described in upgrade step U1 from WRITING-15673.