cmd/clef: change --rpcport to --http.port and update flags in docs (#21318)

# Conflicts:
#	cmd/abigen/main.go
#	cmd/checkpoint-admin/main.go
#	cmd/clef/main.go
#	cmd/ethkey/main.go
#	cmd/evm/main.go
#	cmd/geth/main.go
#	cmd/geth/usage.go
#	cmd/utils/flags.go