Update boilerplate copyright year

[julz]

Although March 2020 was a very long year indeed, it has actually been 2021 for a while now

Eventing and Pkg did this a while ago.

/assign @dprotaso @markusthoemmes

[codecov]

Codecov Report

Merging #11687 (87df6e6) into main (0b09cdd) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #11687      +/-   ##
==========================================
+ Coverage   87.82%   87.86%   +0.04%     
==========================================
  Files         190      190              
  Lines        9249     9249              
==========================================
+ Hits         8123     8127       +4     
+ Misses        876      873       -3     
+ Partials      250      249       -1     

Impacted Files	Coverage Δ
pkg/reconciler/configuration/configuration.go	86.15% <0.00%> (-0.77%)	⬇️
pkg/autoscaler/statforwarder/processor.go	94.44% <0.00%> (+5.55%)	⬆️
pkg/autoscaler/statforwarder/forwarder.go	96.29% <0.00%> (+5.55%)	⬆️

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[psschwei]

/lgtm

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