EKS Upgrades were deliberately disabled by AWS due to an EC2 related issue.
From the AWS status incident:
[11:56 AM PDT] We can confirm that customers are experiencing elevated error rates for cluster upgrade operations in the US-EAST-1 Region.
This is a result of actions we took to prevent additional customer impact following the EC2 network propagation event yesterday in a single Availability Zone (use1-az2).
For EKS customers, we have purposely disabled cluster upgrade operations to prevent any impact to existing clusters and running applications.
We have identified the root cause for the underlying issue and are working towards mitigating the current impact.
We will enable cluster upgrade operations once the issue is resolved.
We will provide an update within the next 60 minutes, or sooner if we have additional information to share.