createGroup can be frontrun

[0xDatapunk]

A malicious actor can monitor calls of createGroup(groupId, admin) and frontrun it with createGroup(groupId, admin1). This prevents new group's creation. If we adopt an incremental groupId, once groupId increases by one each time createGroup is called, we can prevent this from happening

https://github.com/semaphore-protocol/semaphore/blob/8eb19e83fda62644872b2fcfbd85011d3b2c21e2/packages/contracts/contracts/Semaphore.sol#L27 https://github.com/semaphore-protocol/semaphore/blob/8eb19e83fda62644872b2fcfbd85011d3b2c21e2/packages/contracts/contracts/Semaphore.sol#L34