chore(mod): bump to go 1.16, upgrade k8s and knative

[mflendrich]

• Upgrade Go module management to 1.16, to solve the problem with client-go versioning as described here Problems building Go application with client-go kubernetes/client-go#749 (comment) . This removes the necessity for the replace directive 🕺
• Upgrade k8s libs to v0.20.5
• Upgrade knative to v0.21
• Update Go on CI and in Dockerfiles from 1.15 to 1.16

[codecov]

Codecov Report

Merging #1110 (35b8e85) into next (b660668) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next    #1110   +/-   ##
=======================================
  Coverage   56.34%   56.34%           
=======================================
  Files          35       35           
  Lines        3333     3333           
=======================================
  Hits         1878     1878           
  Misses       1316     1316           
  Partials      139      139           

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[shaneutt]

Once we get CI sorted out, LGTM