Remove kubernetes 1.21 variants

[stmcginnis]

Issue number:

Closes #2633

Description of changes:

This removes the 1.21 k8s variants. This version of Kubernetes has gone end of life and we will no longer support it.

Docs and other references to 1.21 have been updated to refer to more current versions.

Note to reviewers

See #2421 and #2316 for examples of past removals of k8s version variants.

Testing done:

cargo make -e BUILDSYS_VARIANT=aws-k8s-1.22 -e BUILDSYS_ARCH=x86_64

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.

[stmcginnis]

Should be ready to review, but marking as draft as we don't want to merge this until we are ready for the next release after the February EOL date.

[stmcginnis]

Rebased on the latest from develop and updated wording for removed variants in the README.

[gthao313]

🚀

[jpmcb]

Looks great - noticed this odd referenced to a 1.21 k8s variant in some test:

https://github.com/stmcginnis/bottlerocket/blob/remove-k8s-1.21/sources/bottlerocket-variant/src/lib.rs#L385-L392

        Test {
            input: "aws-k8s-1.21",
            platform: "aws",
            runtime: "k8s",
            variant_family: "aws-k8s",
            variant_version: Some("1.21"),
            variant_flavor: None,
        },

worth also updating that?

[stmcginnis]

Looks great - noticed this odd referenced to a 1.21 k8s variant in some test:

https://github.com/stmcginnis/bottlerocket/blob/remove-k8s-1.21/sources/bottlerocket-variant/src/lib.rs#L385-L392

        Test {
            input: "aws-k8s-1.21",
            platform: "aws",
            runtime: "k8s",
            variant_family: "aws-k8s",
            variant_version: Some("1.21"),
            variant_flavor: None,
        },

worth also updating that?

I almost did that, but decided I didn't want to set the precedent that we need to remember to bump this every time we raise the minimum k8s version.

Since in this case it is just used as test input and has no actual meaning, it may actually be better to have this refer to a variant that doesn't currently exist to help make it clear that it is just dummy test data.

[stmcginnis]

Switching back over to draft status until everyone is comfortable that we actually can drop 1.21.