[release-1.4] Only proxy some headers from the reply in MTBroker filter

[knative-prow-robot]

This is an automated cherry-pick of #6357

- :broom: MTChannelBroker filter does not proxy headers from replies anymore other than the headers specified in its hardcoded allow-list. The list so far only contains `Retry-After`.

[codecov]

Codecov Report

Merging #6386 (7b47b46) into release-1.4 (cb9a2b5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           release-1.4    #6386   +/-   ##
============================================
  Coverage        82.39%   82.40%           
============================================
  Files              233      233           
  Lines            11562    11567    +5     
============================================
+ Hits              9527     9532    +5     
  Misses            1572     1572           
  Partials           463      463           

Impacted Files	Coverage Δ
pkg/broker/filter/filter_handler.go	75.42% <100.00%> (+0.42%)	⬆️

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[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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