Simplify schema upgrades: remove duplication

[dgageot]

Signed-off-by: David Gageot david@gageot.net

[codecov-io]

Codecov Report

Merging #2212 into master will increase coverage by 0.05%.
The diff coverage is 64.7%.

@@            Coverage Diff            @@
##           master   #2212      +/-   ##
=========================================
+ Coverage   58.55%   58.6%   +0.05%     
=========================================
  Files         188     188              
  Lines        7882    7805      -77     
=========================================
- Hits         4615    4574      -41     
+ Misses       2877    2859      -18     
+ Partials      390     372      -18

Impacted Files	Coverage Δ
pkg/skaffold/schema/v1beta8/upgrade.go	63.63% <0%> (+5.42%)	⬆️
pkg/skaffold/schema/v1beta10/upgrade.go	100% <100%> (+33.33%)	⬆️
pkg/skaffold/schema/v1beta7/upgrade.go	100% <100%> (+33.33%)	⬆️
pkg/skaffold/schema/v1alpha5/upgrade.go	100% <100%> (+29.62%)	⬆️
pkg/skaffold/schema/v1alpha4/upgrade.go	100% <100%> (+36.36%)	⬆️
pkg/skaffold/util/util.go	75.55% <42.85%> (-1.79%)	⬇️

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[priyawadhwa]

lgtm!