Remove the old constant since the new one has been since August 2020

[vagababov]

So 0.18, 0.19 and 0.20 have both labes, enough time now to deprecate the
old one.

/assign mattmoor @dprotaso @tcnghia

[vagababov]

/assign @nak3

[tcnghia]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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• pkg/apis/OWNERS [tcnghia,vagababov]
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[codecov]

Codecov Report

Merging #10559 (a261bd5) into master (89cbb09) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10559      +/-   ##
==========================================
- Coverage   88.04%   88.04%   -0.01%     
==========================================
  Files         186      186              
  Lines        8834     8831       -3     
==========================================
- Hits         7778     7775       -3     
  Misses        812      812              
  Partials      244      244              

Impacted Files	Coverage Δ
pkg/apis/serving/v1/configuration_validation.go	100.00% <100.00%> (ø)
pkg/apis/serving/v1/route_validation.go	97.87% <100.00%> (ø)
pkg/apis/serving/v1/service_validation.go	100.00% <100.00%> (ø)
pkg/reconciler/route/config/domain.go	100.00% <100.00%> (ø)
pkg/reconciler/route/domains/domains.go	100.00% <100.00%> (ø)
pkg/reconciler/route/resources/filters.go	100.00% <100.00%> (ø)
pkg/reconciler/route/resources/labels/labels.go	100.00% <100.00%> (ø)
pkg/reconciler/route/resources/service.go	87.64% <100.00%> (ø)
pkg/autoscaler/statforwarder/forwarder.go	90.74% <0.00%> (-5.56%)	⬇️
pkg/autoscaler/statforwarder/processor.go	83.33% <0.00%> (-5.56%)	⬇️
... and 2 more

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