refactor(x/staking): Migrate UnbondingDelegationByValIndex to use collections

[likhita-809]

Description

Closes: #16385

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[facundomedica]

I'm a bit confused, UnbondingDelegationByValIndex is not being used to Set anywhere, but I think it should be used in SetUnbondingDelegation and RemoveUnbondingDelegation. Also it should not be a collections.Map but a collections.KeySet

[likhita-809]

I'm a bit confused, UnbondingDelegationByValIndex is not being used to Set anywhere, but I think it should be used in SetUnbondingDelegation and RemoveUnbondingDelegation. Also it should not be a collections.Map but a collections.KeySet

Checked with @facundomedica . KeySet is not compatible with this because of the storage format.