Update to Golang 1.21

[creydr]

As per title

[creydr]

/cc @pierDipi @Cali0707 @Leo6Leo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (28c5232) 78.26% compared to head (43f6fad) 78.26%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #331   +/-   ##
=======================================
  Coverage   78.26%   78.26%           
=======================================
  Files           5        5           
  Lines          92       92           
=======================================
  Hits           72       72           
  Misses         19       19           
  Partials        1        1           

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[pierDipi]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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