Update pack image to v0.21.1

[briandealwis]

Update the buildpack publish script to pack v0.21.1.

[aaron-prindle]

LGTM

[codecov]

Codecov Report

Merging #6630 (6ee2620) into main (290280e) will decrease coverage by 0.40%.
The diff coverage is 74.60%.

@@            Coverage Diff             @@
##             main    #6630      +/-   ##
==========================================
- Coverage   70.48%   70.08%   -0.41%     
==========================================
  Files         515      520       +5     
  Lines       23150    23619     +469     
==========================================
+ Hits        16317    16553     +236     
- Misses       5776     5986     +210     
- Partials     1057     1080      +23     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/flags.go	89.00% <0.00%> (-1.82%)	⬇️
cmd/skaffold/skaffold.go	0.00% <ø> (ø)
pkg/diag/recommender/container_errors.go	0.00% <0.00%> (ø)
pkg/diag/validator/pod.go	1.32% <0.00%> (ø)
pkg/skaffold/build/buildpacks/logger.go	0.00% <ø> (ø)
pkg/skaffold/build/cluster/logs.go	0.00% <ø> (-16.67%)	⬇️
pkg/skaffold/build/jib/errors.go	48.93% <50.00%> (ø)
cmd/skaffold/app/cmd/cmd.go	70.32% <66.66%> (-0.73%)	⬇️
pkg/diag/validator/resource.go	47.05% <66.66%> (ø)
pkg/skaffold/build/docker/docker.go	86.44% <66.66%> (-2.85%)	⬇️
... and 75 more

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