docs: Add link to v21 upgrade guide in main README.md (#3468)

Update README.md

Add "Upgrade to v21.x"

Author: szhekpisov-katanox

README.md

@@ -15,6 +15,7 @@ Terraform module which creates Amazon EKS (Kubernetes) resources
   - [Upgrade to v18.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-18.0.md)
   - [Upgrade to v19.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-19.0.md)
   - [Upgrade to v20.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-20.0.md)
+  - [Upgrade to v21.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-21.0.md)
 
 ### External Documentation