Show a warning annotation when "master" branch usage is detected #600

Job	Run time
CI test (make validate)	27s
Code coverage (codecov)	0s
Build moodle-plugin-ci.phar	25s
Integration tests (8.3, main)	6m 14s
Integration tests (8.2, MOODLE_403_STABLE)	6m 4s
Integration tests (8.2, MOODLE_402_STABLE)	6m 6s
Integration tests (8.1, MOODLE_401_STABLE)	6m 51s
Integration tests (8.0, MOODLE_400_STABLE)	6m 29s
Integration tests (8.0, MOODLE_311_STABLE)	5m 38s
Integration tests (7.4, MOODLE_39_STABLE)	5m 36s
Integration tests (PHAR) (8.3, main)	5m 56s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	6m 11s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 22s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 21s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 43s
	1h 13m 23s

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