Prop 73: PoA transition (Plan A): set max_validators to 11

Part of the Proof-of-Authority transition (RFC #70). A standard x/staking MsgUpdateParams — no chain upgrade, effective the moment it passes — setting max_validators from 21 to 11 (the RFC floated 7/11/17). The active set is already 15 and shrinking as validators run at a loss; a smaller curated authority set is cheaper to secure and fund. All other staking params are preserved unchanged. Pairs with the zero-emissions change (votable concurrently).