Migrate some tests from Newtonsoft.Json to System.Text.Json

[JamieMagee]

Part of #231

[github-actions]

👋 Hi! It looks like you modified some files in the Detectors folder.
You may need to bump the detector versions if any of the following scenarios apply:

• The detector detects more or fewer components than before
• The detector generates different parent/child graph relationships than before
• The detector generates different devDependencies values than before

If none of the above scenarios apply, feel free to ignore this comment 🙂

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.3%. Comparing base (8216adf) to head (633f0ab).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1523   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files        418     418           
  Lines      35300   35300           
  Branches    2188    2188           
=====================================
+ Hits       31877   31878    +1     
  Misses      2980    2980           
+ Partials     443     442    -1     

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