[release-1.12] Remove activationLagThreshold to fix 0->1 scaling

[knative-prow-robot]

This is an automated cherry-pick of #3479

KafkaSource KEDA scaling from 0 -> 1 now works even when only a single event is in the topic

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (64635c2) 58.48% compared to head (a3252ca) 61.57%.
Report is 1 commits behind head on release-1.12.

Additional details and impacted files

@@                Coverage Diff                 @@
##             release-1.12    #3481      +/-   ##
==================================================
+ Coverage           58.48%   61.57%   +3.09%     
- Complexity              0      761     +761     
==================================================
  Files                  91      181      +90     
  Lines                9319    12396    +3077     
  Branches                0      265     +265     
==================================================
+ Hits                 5450     7633    +2183     
- Misses               3433     4161     +728     
- Partials              436      602     +166     

Flag	Coverage Δ
java-unittests	70.84% <ø> (?)

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[Cali0707]

/retest-required

[creydr]

/retest

[matzew]

/test reconciler-tests

[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, matzew

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[matzew]

/test reconciler-tests

[Cali0707]

/retest-required