Fixes https://github.com/paritytech/polkadot-sdk/issues/4206

In #1340 one of the storage types was changed from `Candidates` to
`CandidateList`. Since the actual key includes the hash of this value,
all of the candidates stored here are (a) "missing" and (b) unable to
unreserve their candidacy bond.

This migration kills the storage values and refunds the deposit held for
each candidate.

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Signed-off-by: georgepisaltu <[email protected]>
Co-authored-by: georgepisaltu <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: georgepisaltu <[email protected]>