fix claim queue size (#6257)

Reported in
https://github.com/paritytech/polkadot-sdk/issues/6161#issuecomment-2432097120

Fixes a bug introduced in
https://github.com/paritytech/polkadot-sdk/pull/5461, where the claim
queue would contain entries even if the validator groups storage is
empty (which happens during the first session).

This PR sets the claim queue core count to be the minimum between the
num_cores param and the number of validator groups

TODO:
- [x] prdoc
- [x] unit test