Bump minimum supported go version to 1.9.1

[xiangpengzhao]

What this PR does / why we need it:
If #55293 gets merged, we should bump minimum_go_version to 1.9.1

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
ref: #51375 #55293 (comment)

Special notes for your reviewer:
/cc @cblecker @ixdy @luxas

Release note:

The minimum supported go version bumps to 1.9.1.

[cblecker]

/assign

[luxas]

/lgtm
/approve

[luxas]

/retest

Thanks @xiangpengzhao!

[xiangpengzhao]

I think there is sth wrong with CI.
Ref: kubernetes/test-infra#5400

[xiangpengzhao]

@luxas do we need an announcement for developers on the changing of minimum go version supporting? If so, can you help make the announcement please? I guess it's sth like the last announcement: https://groups.google.com/forum/#!msg/kubernetes-dev/0XRRz6UhhTM/YODWVnuDBQAJ

[luxas]

@xiangpengzhao I did so already :) https://groups.google.com/forum/#!topic/kubernetes-dev/INeuA9Flcb8

[xiangpengzhao]

Oh, thanks @luxas ! I didn't check the mailing list when I copyed the previous announcement link :)

[cblecker]

/approve
/retest

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cblecker, luxas, xiangpengzhao

Associated issue: 55293

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 55301, 55319, 54018, 55322, 55125). If you want to cherry-pick this change to another branch, please follow the instructions here.