Fix storage migration of `OffendingValidators` (#4083)

Not all validators in `OffendingValidators` are disabled. This should be
taken in account when migrating the storage item to `DisabledValidators`
(where everyone is disabled).

Also the[ disabling strategy
implementation](https://github.com/paritytech/polkadot-sdk/blob/b71e0dbd

/substrate/frame/staking/src/lib.rs#L1230)
used by the runtime expects a certain number of maximum disabled
validators. The storage migration should guarantee that this limit is
respected.

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Co-authored-by: Maciej <[email protected]>