Update GCP Buildpacks builder image references to :v1

[briandealwis]

Description

GCP Buildpacks has changed to use a versioning scheme via an image tag to avoid breaking consumers from breaking changes. This PR changes all references to gcr.io/buildpacks/builder to add the new :v1 image tag. I updated the relevant pkg/skaffold/schema/ update tests too, though they don't actually matter.

User facing changes (remove if N/A)

• all examples use :v1

[codecov]

Codecov Report

Merging #4313 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4313      +/-   ##
==========================================
+ Coverage   71.93%   71.96%   +0.03%     
==========================================
  Files         322      323       +1     
  Lines       12313    12370      +57     
==========================================
+ Hits         8857     8902      +45     
- Misses       2896     2904       +8     
- Partials      560      564       +4     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/init.go	100.00% <100.00%> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	60.97% <0.00%> (-2.44%)	⬇️
cmd/skaffold/app/cmd/cmd.go	67.64% <0.00%> (-1.89%)	⬇️
pkg/skaffold/config/util.go	71.26% <0.00%> (-0.35%)	⬇️
pkg/skaffold/initializer/init.go	37.20% <0.00%> (ø)
pkg/skaffold/schema/util/util.go	81.81% <0.00%> (ø)
cmd/skaffold/app/cmd/config/list.go	65.21% <0.00%> (ø)
pkg/skaffold/deploy/kubectl/visitor.go	95.83% <0.00%> (ø)
pkg/skaffold/schema/v2beta4/upgrade.go	100.00% <0.00%> (ø)
pkg/skaffold/yaml/yaml.go	81.25% <0.00%> (ø)
... and 5 more

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[briandealwis]

fyi @lukasberger