[Feature] Replace m5.large as default instance type

[butlermg]

What feature/behavior/change do you want?

Some AWS regions do not support m5.large instance types for EKS node groups.
From what we've been able to gather, the t3 family which has broader region availability, and t3.medium is also the default instance type when creating node groups through the EKS console.
We would like to switch eksctl's default instance type for EKS node groups to t3.medium.

Why do you want this feature?

This change will allow eksctl to successfully create EKS clusters with default settings in all regions.

[github-actions]

Hello butlermg 👋 Thank you for opening an issue in eksctl project. The team will review the issue and aim to respond within 1-5 business days. Meanwhile, please read about the Contribution and Code of Conduct guidelines here. You can find out more information about eksctl on our website

[bryantbiggs]

that would be a poor decision. if a region doesn't support the default, the better option is to select a type that is supported. using burstable instances as a default is just a poor decision

[bryantbiggs]

https://github.com/aws/aws-eks-best-practices/blob/mainline/latest/bpg/scalability/data-plane.adoc#use-similar-node-sizes-for-consistent-workload-performance

Avoid instance types that use burstable CPUs like T series instances.