refactor(x/staking): Use gogotypes Int64Value instead of bytes for LastValidatorPower

[likhita-809]

Description

Closes: #XXXX

Here https://github.com/cosmos/cosmos-sdk/pull/17498/files#diff-b751d8b426a3bbcb08ccee5040c1eff9f0ec8d88bde9103506ae1bf8767d6b25R229, the collections map should not have a bytes value but an gogoproto.Int64Value

Initially posted by @testinginprod on slack channel

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[julienrbrt]

lgtm!