Show a warning annotation when "master" branch usage is detected #576

Job	Run time
Code coverage (codecov)	34s
CI test (make validate)	27s
Build moodle-plugin-ci.phar	26s
Integration tests (8.3, main)	6m 22s
Integration tests (8.2, MOODLE_403_STABLE)	6m 9s
Integration tests (8.2, MOODLE_402_STABLE)	5m 55s
Integration tests (8.1, MOODLE_401_STABLE)	6m 48s
Integration tests (8.0, MOODLE_400_STABLE)	6m 37s
Integration tests (8.0, MOODLE_311_STABLE)	6m 4s
Integration tests (7.4, MOODLE_39_STABLE)	5m 34s
Integration tests (PHAR) (8.3, main)	6m 5s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 58s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 34s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 38s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 57s
	1h 15m 8s

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