Migrate to Boosts v2

[nventuro]

We want to migrate our boost contract (PreseededVotingEscrowDelegation) to the new BoostV2 contract developed by Curve. It is not fully compatible with the old one, but that is ok: the important bits (adjusted_balance_of) are the same.

Porting the contract should be relatively straightfoward, since there's no admin, etc. The trickier part is to migrate the currently existing boosts to this new contract. The contract features some migration logic, it'd be good to do a quick overview of it to check if it is compatible with our current scheme.

[jubeira]

Can we mark this as completed? https://github.com/balancer/balancer-deployments/tree/master/tasks/20221205-veboost-v2/output.

The old preseeded delegation doesn't seem to be in use anymore.