Remove v1alpha1 channelable

[vaikas]

Fixes #

Proposed Changes

• 🧹 Remove the v1alpha1 duck types from eventing as they are no longer used anymore, nor have they been used for a long time.
• Preparation for removal of v1beta1 work here and there as well. Not the goal, but while at it did it at some spots.

Release Note

 :broom: v1alpha1 Channel duck types are no longer supported.

Docs

[codecov]

Codecov Report

Merging #5005 (829467f) into master (9089b8e) will increase coverage by 0.44%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #5005      +/-   ##
==========================================
+ Coverage   82.39%   82.84%   +0.44%     
==========================================
  Files         294      288       -6     
  Lines        8391     8009     -382     
==========================================
- Hits         6914     6635     -279     
+ Misses       1073      994      -79     
+ Partials      404      380      -24     

Impacted Files	Coverage Δ
pkg/reconciler/subscription/subscription.go	82.20% <75.00%> (+11.99%)	⬆️
pkg/reconciler/channel/channel.go	72.13% <100.00%> (+0.52%)	⬆️
pkg/reconciler/channel/controller.go	100.00% <100.00%> (ø)
pkg/reconciler/subscription/controller.go	100.00% <100.00%> (ø)
...inmemorychannel/controller/resources/dispatcher.go	100.00% <0.00%> (ø)
pkg/mtbroker/filter/filter_handler.go	80.40% <0.00%> (+1.35%)	⬆️
pkg/reconciler/source/config_watcher.go	79.06% <0.00%> (+18.60%)	⬆️

---

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[n3wscott]

/lgtm
/approve

[vaikas]

/retest

[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/channel/channel.go	77.8%	76.1%	-1.7
pkg/reconciler/subscription/subscription.go	76.6%	86.9%	10.3

[n3wscott]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: n3wscott

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• OWNERS [n3wscott]

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[vaikas]

/retest

flaky comformance tests with rekt