Replace m5.large -> t3.medium as default instance type

[butlermg]

Issue: [Feature] Replace m5.large as default instance type

Description

Some AWS regions do not currently support the default m5.large instance types. Spinning up new node groups in these regions with default settings will result in empty node groups.

The below changes swap the default instance type m5.large with the more modern and widely available t3.medium instance type, which is also the default instance type for node groups created through the EKS console.

Also included are some documentation changes, though these are not necessary.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[github-actions]

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