fix: migration to fix invalid liquidation vault state

[sander2]

The liquidation vault currently has this:

toBeIssuedTokens: 0
issuedTokens: 6,354,070
toBeRedeemedTokens: 6,354,070
collateral: 30,544,076,056,403

This PR decrease toBeRedeemedTokens but not issuedTokens, so that afterwards people can liquidate_redeem. I confirmed that there is indeed an imbalance of 6,354,070 between the vault's issued tokens and the total issuance of kBTC. For future reference, this is how I checked:

  const vaults = await api.query.vaultRegistry.vaults.entries();
  console.log("vault total issued: ", vaults.map((v) => parseInt(v[1].unwrap().issuedTokens.toString())).reduce((a,b) => a+b));
  // compare against tokens.totalIssuance

At current market value, the worth of the locked KSM is 129.8% of the KBTC, so for now that is ok, although the exchange rate will plummet soon.

Also we unlock the vault's funds that got stuck due to the bug in cancel_redeem