Deprecating EnvTemplate's use of {{.IMAGE_NAME}}

[felixtran39]

Description
This is the coding logic and additional test cases for deprecating envTemplate's use of {{.IMAGE_NAME}} in its template. Users should be using envTemplate to indicate solely the tag value for their images.

User facing changes)
Users will now see a warning that {{.IMAGE_NAME}} is deprecated when using {{.IMAGE_NAME}} in their envTemplate tagger.
The warning will look like
{{.IMAGE_NAME}} is deprecated, envTemplate's template should only specify the tag value. See https://skaffold.dev/docs/pipeline-stages/taggers/

Follow-up Work
Changes to docs to accompany this deprecation: #4532

[codecov]

Codecov Report

Merging #4533 into master will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4533      +/-   ##
==========================================
+ Coverage   72.40%   72.72%   +0.32%     
==========================================
  Files         334      334              
  Lines       12965    13006      +41     
==========================================
+ Hits         9387     9459      +72     
+ Misses       2982     2950      -32     
- Partials      596      597       +1     

Impacted Files	Coverage Δ
pkg/skaffold/build/tag/tag.go	100.00% <100.00%> (ø)
pkg/skaffold/kubernetes/client.go	0.00% <0.00%> (-40.00%)	⬇️
pkg/skaffold/kubernetes/context/context.go	85.36% <0.00%> (-12.20%)	⬇️
pkg/skaffold/deploy/labels.go	49.36% <0.00%> (-6.59%)	⬇️
pkg/skaffold/deploy/kubectl/labels.go	87.09% <0.00%> (-6.01%)	⬇️
pkg/skaffold/schema/util/util.go	76.66% <0.00%> (-5.16%)	⬇️
pkg/skaffold/deploy/kubectl/cli.go	88.63% <0.00%> (-2.28%)	⬇️
cmd/skaffold/app/cmd/cmd.go	64.96% <0.00%> (-1.46%)	⬇️
pkg/skaffold/deploy/deploy_mux.go	87.80% <0.00%> (-1.33%)	⬇️
pkg/skaffold/deploy/kustomize.go	77.71% <0.00%> (-0.63%)	⬇️
... and 27 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a988b46...d39652a. Read the comment docs.

[yuwenma]

LGTM.

[briandealwis]

Otherwise LGTM.