Update Golang version to 1.21.7; update aws-vpc-cni chart README

[jdn5126]

What type of PR is this?
dependencies

Which issue does this PR fix:
N/A

What does this PR do / Why do we need it:
This PR updates the Golang build version to 1.21.7. This is done as part of a regular upgrade.

This PR also adds a command to the aws-vpc-cni chart README to help customers trying to adopt an existing VPC CNI installation into Helm.

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:
N/A

Testing done on this change:
Manually verified that image builds and can be installed on a cluster.

Will this PR introduce any new dependencies?:
No

Will this break upgrades or downgrades? Has updating a running cluster been tested?:
No, Yes

Does this change require updates to the CNI daemonset config files to work?:
No

Does this PR introduce any user-facing change?:
Yes

Update Golang version to 1.21.7

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.