Set XCMP Queue Drop Threshold

We currently default to an XCMP drop threshold of `5` which might lead to dropped XCMP messages in case the chain stalls.
We want to change the threshold to the maximum XCMP channel size, so that no messages are dropped in case of stall.

We should also choose a suspend threshold. Something low like 10 or 20 might be good, as we don't expect to have a backlog of XCMP messages.

### Todos
- [ ] create governance call to adjust storage
- [ ] write chopsticks test that verifies it will work fine

Start by locating the XCMP queue configuration and the storage setting affected by the threshold change. Review the governance-call requirements, then create the requested Chopsticks test to verify behavior during a chain stall. Done means the drop threshold uses the maximum channel size, a suspend threshold is selected, storage adjustment is governed, and the test passes.

Written by the indexing model from the issue text.