Nimbus updates critical issue after mainnet outage on all Docker images

Nimbus consensus client v26.2.1 is a medium-urgency update for ARM and multiarch Docker images, addressing the cause of Nimbus outage on mainnet present in v26.1.0 and earlier. Those running v26.1.0 or earlier should verify they're on the canonical chain, restart Nimbus otherwise, and update to 26.2.1. No change to beacon node or validator client between v26.2.0 and v26.2.1. Update "sometime within our normal two-week cycle".