Remove copyright YEAR from generated Go files

[bmelbourne]

What this PR does / why we need it:
Fixes the following issue in kops CI test make quick-ci related to the change to Copyright 2021 in the Go boilerplate headers.

k8s.io/code-generator/cmd/client-gen
# These code-generator tools still depend on the kops repo being in GOPATH
# ref: https://github.com/kubernetes/gengo/issues/64
/home/runner/work/kops/kops/go/src/k8s.io/kops/_output/bin/conversion-gen --verify-only --skip-unsafe=true --input-dirs k8s.io/kops/pkg/apis/kops/v1alpha2 --v=0  --output-file-base=zz_generated.conversion \
	 --go-header-file "hack/boilerplate/boilerplate.go.txt"
F0101 13:16:16.689939   18295 main.go:134] Error: Failed executing generator: some packages had errors:
errors in package "k8s.io/kops/pkg/apis/kops/v1alpha2":
output for "v1alpha2/zz_generated.conversion.go" differs; first existing/expected diff: 
  "0 The Kubernetes Authors.\n\nLicensed under the Apache License, Version 2.0 (the \"License\");\nyou may n"
  "1 The Kubernetes Authors.\n\nLicensed under the Apache License, Version 2.0 (the \"License\");\nyou may n"

Similar issue kubernetes/kubernetes#59172 already addressed in Kubernetes project back in 2018.

Which issue(s) this PR fixes
Fixes #10515

Special notes for your reviewer:
hack/boilerplate/boilerplate.py has been copied from Kubernetes as this project uses a similar approach to re-generating Go boilerplate headers and has been well tested for several years.

https://github.com/kubernetes/kubernetes/blob/master/hack/boilerplate/boilerplate.py

[k8s-ci-robot]

Hi @bmelbourne. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[bmelbourne]

/assign @KashifSaadat

[bmelbourne]

@rifelpet
Can you provide an /ok-to-test please?

[hakman]

/ok-to-test

[rifelpet]

Thanks for fixing this! I'm glad that python script will be more in line with k/k https://github.com/kubernetes/kubernetes/blob/cea1d4e20b4a7886d8ff65f34c6d4f95efcb4742/hack/boilerplate/boilerplate.py

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bmelbourne, rifelpet

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