maxSize field from addShard command has been deprecated and InvalidOptions error is thrown in case user runs addShard

.
maxSize field entry won't be generated in config.shards, and in case is present because of an upgrade from previous version (we do not delete the entry when upgrade) the field is ignored.
Every cluster related stats printing the max size of the shard do not provide this information anymore.

Description of Linked Ticket

Get rid of maxSize since it's a soft constraint and doesn't even do well what it's intended to do: it's a legacy parameter that made sense with the MMAP storage engine. Nowadays, the WT storage engine stores data in compressed form so this parameter is not honored anymore.

Behavioral changes and upgrade/downgrade considerations:

• In versions >= 6.1 if you specify maxSize on addShard we will error with “InvalidOptions”

• maxSize configuration field in config.settings will be kept until 7.0 to support dowgrade and then removed completely as part of 7.1 FCV upgrade procedure (SERVER-68430)