Added missing php83 branch in self-update tests #624

Job	Run time
CI test (make validate)	54s
Code coverage (codecov)	0s
SelfUpdate tests (PHAR) (8.3)	32s
SelfUpdate tests (PHAR) (8.2)	40s
SelfUpdate tests (PHAR) (8.1)	45s
SelfUpdate tests (PHAR) (8.0)	26s
SelfUpdate tests (PHAR) (7.4)	8s
Build moodle-plugin-ci.phar	25s
Integration tests (8.3, main)	6m 24s
Integration tests (8.2, MOODLE_403_STABLE)	6m 11s
Integration tests (8.2, MOODLE_402_STABLE)	5m 53s
Integration tests (8.1, MOODLE_401_STABLE)	6m 34s
Integration tests (8.0, MOODLE_400_STABLE)	6m 37s
Integration tests (8.0, MOODLE_311_STABLE)	6m 10s
Integration tests (7.4, MOODLE_39_STABLE)	5m 36s
Integration tests (PHAR) (8.3, main)	5m 46s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 26s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 42s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 23s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 52s
	1h 16m 24s

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