Drop k8s 1.28 variants

[koooosh]

Issue number:

#4706

Description of changes:

Drop the aws-k8s-1.28, aws-k8s-1.28-nvidia, aws-k8s-1.28-fips, vmware-k8s-1.28, and vmware-k8s-1.28-fips variants.

Testing done:

cargo make succeeds with defaults, while cargo make -e BUILDSYS_VARIANT={VARIANT} fails with all dropped variants above, example:

[cargo-make][1] INFO - Running Task: build-variant
error: manifest path `variants/aws-k8s-1.28-nvidia/Cargo.toml` does not exist
Error while executing command, exit code: 101
Error: Command was unsuccessful, exit code 105
Error while executing command, exit code: 1

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.