Show a warning annotation when "master" branch usage is detected #602

Job	Run time
CI test (make validate)	30s
Code coverage (codecov)	0s
Build moodle-plugin-ci.phar	35s
Integration tests (8.3, main)	6m 9s
Integration tests (8.2, MOODLE_403_STABLE)	6m 0s
Integration tests (8.2, MOODLE_402_STABLE)	5m 51s
Integration tests (8.1, MOODLE_401_STABLE)	7m 15s
Integration tests (8.0, MOODLE_400_STABLE)	6m 23s
Integration tests (8.0, MOODLE_311_STABLE)	6m 26s
Integration tests (7.4, MOODLE_39_STABLE)	5m 57s
Integration tests (PHAR) (8.3, main)	5m 46s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 46s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 18s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 33s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 51s
	1h 14m 20s

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