Resolves https://github.com/paritytech/polkadot-sdk/issues/5929

Migrates `ChildBountyDescriptions` to be indexed instead of unique child
bounty id unique per all child bounties in the pallet to be unique per
every parent bounty.

Migrates `(ParentBounty, ChildBounty)` keys inside `ChildBounties`
storage item to use new `ChildBounty` ids starting from `0`.

@paritytech/frame-coders

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: DavidK <davidk@parity.io>
Co-authored-by: muharem <ismailov.m.h@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>