fix(x/staking): DelegationsByValidator migrations

[atheeshp]

Description

we have found a bug earlier that we are using the same prefix key for UnbondingIDKey & DelegationByValIndexKey ref and that's been addressed in #16176, but we missed to update the key in staking migrations.

Closes: #XXXX

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[github-actions]

@atheeshp your pull request is missing a changelog!

[likhita-809]

lgtm

[julienrbrt]

Good find!