DesiredCandidates lower than MinCandidates

[lazam]

We encounter the scenario in Rococo Assethub which was initially set to DesiredCandidates: 1. You can set the DesiredCandidates lower than MinCandidates but the collator can't be removed due to TooFewCandidates. It will be better to have a check and reject the call if the DesiredCandidates <= MinCandidates.

[bkchr]

Not sure this makes sense. The MinCandidates are compared against the candidates + invulnerable. So, it is totally fine to say that desired_candidates == 1 and having min candidates of 2 and then having an invulnerable of 1.

[joepetrowski]

Rococo AH is still on v9430, which is before the change from MinCandidates to MinCollators, so this should already be sorted by #2801

https://github.com/paritytech/cumulus/blob/release-parachains-v9430/pallets/collator-selection/src/lib.rs#L402