Output `.json` by default in C3 #6330

Job	Run time
Checks	0s
Add PR to project	2s
Checks	1m 16s
Tests (macos-13, ./fixtures/*)	1m 36s
Tests (macos-13, ./packages/*)	2m 26s
Tests (ubuntu-latest, ./fixtures/*)	40s
Tests (ubuntu-latest, ./packages/*)	50s
Tests (ubuntu-latest, ./tools)	42s
Tests (windows-latest, ./fixtures/*)	1m 44s
Tests (windows-latest, ./packages/*)	1m 47s
	11m 3s

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