chore: Update documentation related to default EKS node group settings and v18.x security group changes

[bryantbiggs]

Description

• update documentation to correct example usage of default EKS managed node group
  • by default, the module creates a launch template to ensure tags are propagated to instances, etc., so users may need to disable it to use the default template provided by the AWS EKS managed node group service when setting EKS managed node service settings that are not required to be set in a launch template (its a weird/confusing mix but the way the AWS service works today)
• add a note in the backwards incompatible changes section regarding security groups to ensure users reading the upgrade guide are notified of the security group changes (ref: Cannot create Ingress resources after upgrading to 18 release #1748)
• update eks-managed-node-group example to correct remote SSH access to use actual key; errors out otherwise
  • add example of using gravitron type instance (ref: instance t4g not available #1734)

Motivation and Context

• Closes instance t4g not available #1734
• Closes The disk_size does not changed. #1739
• Ref: Cannot create Ingress resources after upgrading to 18 release #1748

Breaking Changes

• No

How Has This Been Tested?

• I have tested and validated these changes using one or more of the provided examples/* projects
  - validated with eks-managed-node-group example

[antonbabenko]

This PR is included in version 18.0.6 🎉

[github-actions]

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