Update Kops Go build supported versions 1.15

[bmelbourne]

Update Kops documentation to reflect support for Go versions go1.15+.

Kops make verify and make all examples test successfully tested against Go versions go1.15 and go1.15.3.

Latest Go version go1.13.15 is no longer supported due to the following error...

$ go version
go version go1.13.15 linux/amd64

$ make clean && make apimachinery
if test -e /c/GitHub/kops/.build; then rm -rfv /c/GitHub/kops/.build; fi
bazel clean
INFO: Starting clean (this may take a while). Consider using --async if the clean takes more than several minutes.
rm -rf tests/integration/update_cluster/*/.terraform
sh -c hack/make-apimachinery.sh
-mod=mod not supported (can be '', 'readonly', or 'vendor')
make: *** [Makefile:526: apimachinery-codegen] Error 1

[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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[hakman]

That is quite a good change. Always forget to change here. Could we say just that build requires 1.15?
Would avoid confusions when some generate / update / test command fails because of 1.14. We don't test anymore with 1.14 at all.

Suggested change

	- building kops requires go 1.14 or 1.15
	- building kops requires go 1.15

[bmelbourne]

Works for me. Although, I didn't experience any build issues with version go1.14, I'm always happy to go with the latest version!

[hakman]

Thanks. There are a few hack/verify-* and hack/update-* commands that don't work with 1.14. Better not confuse people.

[hakman]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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