Update VC to submit beacon committee selections once per epoch #7718
Description
Describe the bug
Something that's been missed during the beacon committee selection submissions is that they are being sent once per slot, instead of once per epoch, at the beginning of the epoch. It generally works if only Nimbus VC is used in a DV cluster or another VC that has the same issue (Lodestar and Lighthouse are such).
BN Spec - https://ethereum.github.io/beacon-APIs/#/Validator/submitBeaconCommitteeSelections
To Reproduce
Running a DV cluster with 1/2 nodes running Nimbus and 1/2 nodes running Teku. Aggregations are failing.
Additional context
Add any other context about the problem here.