Add PHP 8.3 support (#20) #38

Job	Run time
PHPSpec tests (lowest, 8.1, ubuntu-latest)	11s
PHPSpec tests (lowest, 8.1, windows-latest)	32s
PHPSpec tests (lowest, 8.2, ubuntu-latest)	15s
PHPSpec tests (lowest, 8.2, windows-latest)	39s
PHPSpec tests (lowest, 8.3, ubuntu-latest)	14s
PHPSpec tests (lowest, 8.3, windows-latest)	31s
PHPSpec tests (highest, 8.1, ubuntu-latest)	16s
PHPSpec tests (highest, 8.1, windows-latest)	34s
PHPSpec tests (highest, 8.2, ubuntu-latest)	10s
PHPSpec tests (highest, 8.2, windows-latest)	33s
PHPSpec tests (highest, 8.3, ubuntu-latest)	12s
PHPSpec tests (highest, 8.3, windows-latest)	26s
PHPSpec tests (locked, 8.1, ubuntu-latest)	16s
PHPSpec tests (locked, 8.1, windows-latest)	32s
PHPSpec tests (locked, 8.2, ubuntu-latest)	20s
PHPSpec tests (locked, 8.2, windows-latest)	35s
PHPSpec tests (locked, 8.3, ubuntu-latest)	27s
PHPSpec tests (locked, 8.3, windows-latest)	30s
	7m 13s

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