Freeze v1beta8 skaffold config

[corneliusweig]

There are upcoming config changes.

[codecov-io]

Codecov Report

Merging #1965 into master will increase coverage by 0.06%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #1965      +/-   ##
==========================================
+ Coverage   52.52%   52.59%   +0.06%     
==========================================
  Files         180      182       +2     
  Lines        7865     7893      +28     
==========================================
+ Hits         4131     4151      +20     
- Misses       3348     3352       +4     
- Partials      386      390       +4

Impacted Files	Coverage Δ
pkg/skaffold/schema/versions.go	69.23% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta7/upgrade.go	66.66% <ø> (ø)	⬆️
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/v1beta8/config.go	100% <100%> (ø)
pkg/skaffold/schema/v1beta8/upgrade.go	66.66% <66.66%> (ø)

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[nkubala]

thanks!