Show a warning annotation when "master" branch usage is detected #606

Job	Run time
CI test (make validate)	39s
Code coverage (codecov)	0s
Build moodle-plugin-ci.phar	32s
Integration tests (8.3, main)	6m 30s
Integration tests (8.2, MOODLE_403_STABLE)	6m 57s
Integration tests (8.2, MOODLE_402_STABLE)	6m 16s
Integration tests (8.1, MOODLE_401_STABLE)	7m 22s
Integration tests (8.0, MOODLE_400_STABLE)	6m 43s
Integration tests (8.0, MOODLE_311_STABLE)	5m 59s
Integration tests (7.4, MOODLE_39_STABLE)	6m 6s
Integration tests (PHAR) (8.3, main)	6m 3s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	6m 25s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	6m 6s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 37s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	6m 21s
	1h 18m 36s

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