[release-1.3] 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

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@@              Coverage Diff               @@
##             release-1.3    #6387   +/-   ##
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  Coverage               ?   82.19%           
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  Files                  ?      231           
  Lines                  ?     7789           
  Branches               ?        0           
==============================================
  Hits                   ?     6402           
  Misses                 ?      937           
  Partials               ?      450           

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[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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