Upgrade libraries to k8s 1.20.

[willbeason]

I have not been able to test this locally. Everything builds, but:

cert-controller$ make test
go test ./... -coverprofile cover.out
2021/02/09 07:37:55 failed to start the controlplane. retried 5 times: fork/exec /usr/local/kubebuilder/bin/etcd: exec format error
FAIL	github.com/open-policy-agent/cert-controller/pkg/rotator	0.138s
FAIL
make: *** [Makefile:7: test] Error 1

I haven't found any setup instructions on how to successfully run tests. I get the same issue when trying to run tests on HEAD.

[codecov-io]

Codecov Report

Merging #31 (83600a1) into master (4842e47) will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   51.74%   52.27%   +0.53%     
==========================================
  Files           1        1              
  Lines         373      373              
==========================================
+ Hits          193      195       +2     
+ Misses        124      123       -1     
+ Partials       56       55       -1     

Flag	Coverage Δ
unittests	52.27% <ø> (+0.53%)	⬆️

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Impacted Files	Coverage Δ
pkg/rotator/rotator.go	52.27% <0.00%> (+0.53%)	⬆️

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[maxsmythe]

This LGTM

@sozercan LGTY?

[sozercan]

LGTM. Should we investigate if there are any roadblocks for vendoring in GK before merging?

[maxsmythe]

I don't think it's necessary, we should upgrade Gatekeeper's dependencies regardless, and it seems selfish to block on 1 project without strong reason.