[Staking] Fix: migrate_currency supports cases with leftover old staking locks

[Ank4n]

closes #7931.

TODO

• Rerun migrate_currency for all accounts with current state of testnet/mainnets in a remote externality test.

Migration stats:

Westend: success: 73023, err: 0, total force withdrawn stake: 340726297547707, no_migration_needed: 53
Kusama: success: 26096, err: 0, total force withdrawn stake: 893395700711867, no_migration_needed: 0
Polkadot: success: 56044, err: 0, total force withdrawn stake: 157489603546239, no_migration_needed: 0

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13881839484
Failed job name: fmt

[kianenigma]

The largest account that has to force_withdraw around 150 DOTs is https://polkadot.subscan.io/account/13YMK2eb11XSc7CJgqDBrNswzLpMMc8pWkzxYkqaMruPG8xk which is a parachain sovereign account. It makes sense because they have a lot of reserve.

What I ask here is to make sure to post a final list of these accounts right before the fellowship runtimes upgrade to 2503 in the runtimes PR for transparency.

[kianenigma]

I have edited the test to print a few more stats + pass the try-state checks (by bringing in the VoterList and other pallets).

LGTM