Backport of fix Pools 6->7 migration (#2942) (#3094)

This should result as a patched `24.0.1` version for
https://crates.io/crates/pallet-nomination-pools/24.0.0.

Relates to: https://github.com/polkadot-fellows/runtimes/pull/137

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Alexander Samusev <[email protected]>
Co-authored-by: Dónal Murray <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>
Co-authored-by: PG Herveou <[email protected]>