Add PHP 8.3 support (#20) #41

Job	Run time
PHPSpec tests (lowest, 8.0, ubuntu-latest)	12s
PHPSpec tests (lowest, 8.0, windows-latest)	28s
PHPSpec tests (lowest, 8.1, ubuntu-latest)	17s
PHPSpec tests (lowest, 8.1, windows-latest)	38s
PHPSpec tests (lowest, 8.2, ubuntu-latest)	12s
PHPSpec tests (lowest, 8.2, windows-latest)	39s
PHPSpec tests (lowest, 8.3, ubuntu-latest)	12s
PHPSpec tests (lowest, 8.3, windows-latest)	38s
PHPSpec tests (highest, 8.0, ubuntu-latest)	14s
PHPSpec tests (highest, 8.0, windows-latest)	37s
PHPSpec tests (highest, 8.1, ubuntu-latest)	16s
PHPSpec tests (highest, 8.1, windows-latest)	36s
PHPSpec tests (highest, 8.2, ubuntu-latest)	16s
PHPSpec tests (highest, 8.2, windows-latest)	38s
PHPSpec tests (highest, 8.3, ubuntu-latest)	14s
PHPSpec tests (highest, 8.3, windows-latest)	39s
PHPSpec tests (locked, 8.0, ubuntu-latest)	14s
PHPSpec tests (locked, 8.0, windows-latest)	35s
PHPSpec tests (locked, 8.1, ubuntu-latest)	8s
PHPSpec tests (locked, 8.1, windows-latest)	35s
PHPSpec tests (locked, 8.2, ubuntu-latest)	11s
PHPSpec tests (locked, 8.2, windows-latest)	35s
PHPSpec tests (locked, 8.3, ubuntu-latest)	27s
PHPSpec tests (locked, 8.3, windows-latest)	38s
	10m 9s

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