Use ValidatorUpdates in x/staking and x/gentutil

[julienrbrt]

#19735 migrated the legacy HasABCIGenesis and HasABCIEndblock api to use appmodule.ValidatorUpdates.

Only staking and genutil are implementing these interfaces. After #19735 we should update x/gentutil and x/staking keeper to directly return ValidatorUpdates in their keeper instead of doing the conversion in their module.go.

[chixiaowen]

let me try @julienrbrt