[release-1.4] Mark --inject-broker flag as deprecated

[knative-prow-robot]

This is an automated cherry-pick of #1661

[codecov]

Codecov Report

Merging #1665 (6ef7f05) into release-1.4 (ff097e9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           release-1.4    #1665   +/-   ##
============================================
  Coverage        79.48%   79.49%           
============================================
  Files              171      171           
  Lines            12963    12969    +6     
============================================
+ Hits             10304    10310    +6     
  Misses            1941     1941           
  Partials           718      718           

Impacted Files	Coverage Δ
pkg/kn/commands/trigger/update_flags.go	100.00% <100.00%> (ø)

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[dsimansk]

@rhuss FYI cherry-pick PR to release-1.4.

[dsimansk]

/approve
/lgtm

[knative-prow]

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