Move all "master" uses to "main" #524

Job	Run time
CI test (make validate)	30s
Code coverage (codecov)	27s
Integration tests (8.2, main)	5m 23s
Build moodle-plugin-ci.phar	39s
Integration tests (8.2, MOODLE_403_STABLE)	5m 17s
Integration tests (8.2, MOODLE_402_STABLE)	5m 25s
Integration tests (8.1, MOODLE_401_STABLE)	6m 5s
Integration tests (8.0, MOODLE_400_STABLE)	5m 52s
Integration tests (8.0, MOODLE_311_STABLE)	5m 19s
Integration tests (7.4, MOODLE_310_STABLE)	5m 19s
Integration tests (7.4, MOODLE_39_STABLE)	5m 7s
Integration tests (7.4, MOODLE_38_STABLE)	5m 23s
Integration tests (PHAR) (8.2, main)	5m 20s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	4m 40s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	5m 57s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 3s
	1h 11m 46s

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