add 1.21 in make file

[ravisinha0506]

Issue #, if available:

Description of changes:
Adding 1.21 in the makefile.

Testing

Able to create 1.21 ami using make 1.21 command:

    amazon-ebs: AMI: ami-00c3e475efaa91791
==> amazon-ebs: Waiting for AMI to become ready...
==> amazon-ebs: Modifying attributes on AMI (ami-00c3e475efaa91791)...
    amazon-ebs: Modifying: description
==> amazon-ebs: Modifying attributes on snapshot (snap-0e525062e2163df11)...
==> amazon-ebs: Adding tags to AMI (ami-00c3e475efaa91791)...
==> amazon-ebs: Tagging snapshot: snap-0e525062e2163df11
==> amazon-ebs: Creating AMI tags
    amazon-ebs: Adding tag: "source_ami_id": ""
    amazon-ebs: Adding tag: "kubernetes": "1.21.2/2021-07-05/bin/linux/x86_64"
    amazon-ebs: Adding tag: "cni_plugin_version": "v0.8.6"
    amazon-ebs: Adding tag: "Name": "amazon-eks-node-1.21-v20210721"
    amazon-ebs: Adding tag: "created": "1626893467"
    amazon-ebs: Adding tag: "docker_version": "19.03.13ce-1.amzn2"
    amazon-ebs: Adding tag: "containerd_version": "1.4.6-2.amzn2"
==> Builds finished. The artifacts of successful builds are:
--> amazon-ebs: AMIs were created:
us-west-2: ami-00c3e475efaa91791

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