Remove ext.json from composer.json

[OskarStark]

https://php.watch/versions/8.0/ext-json

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (67d438a) 92.52% compared to head (fd3038e) 92.52%.

❗ Current head fd3038e differs from pull request most recent head 4e2c536. Consider uploading reports for the commit 4e2c536 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #388   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          15       15           
  Lines         348      348           
=======================================
  Hits          322      322           
  Misses         26       26           

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[DamienHarper]

Replaced by #393