Reward pool migration fix (#13715)

* reward pool migration fix

* comment

* remove generic

* rm max

* foramtting

* Add note to V4 migration

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add more asserts to the V5 migration

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Make compile

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update frame/nomination-pools/src/migration.rs

Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>

* Make V4 migration re-usable

Otherwise it wont chain together with the V5.

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add MigrateV3ToV5 wrapper

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>