Support KReference.Group in Subscription.Spec.Channel

[slinkydeveloper]

Fixes #5086

Proposed Changes

• 🎁 KReference.Group now can be used in Subscription.Spec.Channel as well

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact: N/A
• Spec PR for any new API feature: N/A
• Conformance test for any change to the spec: N/A

Release Note

:gift: `KReference.Group` now can be used in `Subscription.Spec.Channel` as well

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

Merging #5520 (0ce949d) into main (6adafe5) will decrease coverage by 0.05%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##             main    #5520      +/-   ##
==========================================
- Coverage   82.62%   82.56%   -0.06%     
==========================================
  Files         198      198              
  Lines        6117     6125       +8     
==========================================
+ Hits         5054     5057       +3     
- Misses        737      741       +4     
- Partials      326      327       +1     

Impacted Files	Coverage Δ
pkg/reconciler/subscription/subscription.go	79.05% <37.50%> (-1.36%)	⬇️

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[matzew]

LGTM

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/subscription/subscription.go	85.9%	85.3%	-0.5

[lionelvillard]

/lgtm

/cc @vaikas @n3wscott to make sure you see this.