PodAutoscaler Active Condition should not affect Reachability

[dprotaso]

Fixes #14115

PodAutoscaler.Spec.Reachability was added a long time ago. Since then we've added a ScaleTargetInitialized condition to make it easier to know if we've scaled up from zero successfully - and this simplified various condition checks in the autoscaler etc.

Given that the PodAutoscaler's Active condition would toggle Reachability as @SaschaSchwarze0 pointed out.

I believe we don't need this circular logic - reachability should be set depending on the revision's routing state with the exception of failing to progress the deployment/start a users container.

[dprotaso]

Besides the feedback loop with active - reachability is also set to unreachable when the revision's deployment is known to be in a failure state (eg. progress deadline hit/imagepullbackoff)

I'm not sure that's necessary because it seems like reachability just affects scale bounds and indirectly KPA condition status

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04% ⚠️

Comparison is base (ad5455e) 86.16% compared to head (6e94600) 86.12%.
Report is 42 commits behind head on main.

❗ Current head 6e94600 differs from pull request most recent head 1d9f1f5. Consider uploading reports for the commit 1d9f1f5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14309      +/-   ##
==========================================
- Coverage   86.16%   86.12%   -0.04%     
==========================================
  Files         195      196       +1     
  Lines       14702    14787      +85     
==========================================
+ Hits        12668    12736      +68     
- Misses       1729     1745      +16     
- Partials      305      306       +1     

Files Changed	Coverage Δ
pkg/reconciler/revision/resources/pa.go	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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[SaschaSchwarze0]

@dprotaso I have taken your PR and tested the problematic scenario and simple scaleUp and scaleFrom/ToZero. Is all working. I can close my PR if consensus exists to go your route. Thanks for checking.

[dprotaso]

/assign @nak3 @ReToCode @skonto

cc @jsanin-vmw who's tackling a similar issue where pods don't scale down when we don't have metrics

[nak3]

One minor comment otherwise LGTM.

[nak3]

/lgtm
/approve

Thank you!

[knative-prow]

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