fix: enforce quorum registration on churn

[ypatil12]

NOTE: #464 was merged to wrong target

Motivation:

It is possible to break the maxOperatorCount invariant by doing the following:

Let's assume there are two quorums, 1 and 2, with a maxOperatorCount of 2.

1. Alice & Bob register for quorum 1
2. Bob registers for quorum 2
3. Bob deregisters from quorum 1, Charlie enters
4. Quorum 1 Members: Alice/Charlie. Quorum 2 members: Bob
5. Eve creates a churn registration that exits Bob. Quorum 1 has 3 members. This works just fine since we allow a churn to occur if the operatorToKick is registered to the AVS (not the quorum):

   eigenlayer-middleware/src/SlashingRegistryCoordinator.sol

   Lines 405 to 408 in f5adbca

   	// Check if operator is registered for all quorums we're trying to remove them from
   	if (quorumsToRemove.isSubsetOf(currentBitmap)) {
   	_forceDeregisterOperator(operator, quorumNumbers);
   	}

Although this assumes that the churnApprover is buggy, we should still be enforcing that you are churning a user if they are registered for the quorum.

Modifications:

Require that the operatorToKick is registered for the quorum.

Result:

Stricter churn guarantees.