Remove redundant ext-json requirement

[fain182]

Since PHP 8.0, ext-json is bundled into the PHP core and cannot be disabled. Given that the project already requires php ^8.0, declaring ext-json as a separate dependency is unnecessary.

Fixes #612

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (50d789a) to head (a679cc2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #613   +/-   ##
=========================================
  Coverage     98.32%   98.32%           
  Complexity      689      689           
=========================================
  Files            86       86           
  Lines          1968     1968           
=========================================
  Hits           1935     1935           
  Misses           33       33           

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