Update docs with Ubuntu 20.04 supported in EKS <= 1.29 (#7618)

docs: ubuntu 20.04 supported in EKS <= 1.29

This adds a note that ubuntu focal (20.04) is supported up to EKS 1.29

Author: rpocase

userdocs/src/usage/custom-ami-support.md

@@ -56,7 +56,7 @@ The `--node-ami-family` can take following keywords:
 |--------------------------------|:--------------------------------------------------------------------------------------------------------------:|
 | AmazonLinux2                   | Indicates that the EKS AMI image based on Amazon Linux 2 should be used (default).                             |
 | Ubuntu2204                     | Indicates that the EKS AMI image based on Ubuntu 22.04 LTS (Jammy) should be used (available for EKS >= 1.29). |
-| Ubuntu2004                     | Indicates that the EKS AMI image based on Ubuntu 20.04 LTS (Focal) should be used.                             |
+| Ubuntu2004                     | Indicates that the EKS AMI image based on Ubuntu 20.04 LTS (Focal) should be used (supported for EKS <= 1.29). |
 | Ubuntu1804                     | Indicates that the EKS AMI image based on Ubuntu 18.04 LTS (Bionic) should be used.                            |
 | Bottlerocket                   | Indicates that the EKS AMI image based on Bottlerocket should be used.                                         |
 | WindowsServer2019FullContainer | Indicates that the EKS AMI image based on Windows Server 2019 Full Container should be used.                   |