swap cjson dependency

[bobcallaway]

This removes the dependency on github.com/tent/canonical-json-go which is no longer maintained.

[codecov]

Codecov Report

Merging #1441 (82de864) into main (fb50ad8) will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1441      +/-   ##
==========================================
+ Coverage   64.32%   64.38%   +0.05%     
==========================================
  Files          82       82              
  Lines        7874     7873       -1     
==========================================
+ Hits         5065     5069       +4     
+ Misses       2161     2157       -4     
+ Partials      648      647       -1     

Flag	Coverage Δ
e2etests	47.70% <66.66%> (+0.01%)	⬆️
unittests	43.25% <33.33%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/pki/tuf/tuf.go	53.26% <66.66%> (-0.51%)	⬇️

... and 2 files with indirect coverage changes

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[lukehinds]

nice, thanks!