Do not call main chain as mainnet

[cthulhu-rider]

• inspired by docs: rename morph to fschain #3038

NeoFS runs above two chains - main and FS ones. And there is a public NeoFS network - NeoFS Mainnet. Sometimes main chain-related stuff is called mainnet. Like here

neofs-node/config/example/ir.yaml

Line 99 in 474d541

mainnet:

this is legit for NeoFS Mainnet itself, but confuses me in general

i propose to call main chain as main chain (mainchain where appropriate) everywhere where it is not NeoFS Mainnet itself that is meant (like https://github.com/nspcc-dev/neofs-node/tree/master/config/mainnet). With this, there will be main and FS chains pair instead of mainnet and FS chain

old configs must keep working ofc

[roman-khimov]

Related to #2702/#3021, makes sense.