Show a warning annotation when "master" branch usage is detected #579

Job	Run time
Code coverage (codecov)	25s
CI test (make validate)	59s
Build moodle-plugin-ci.phar	49s
Integration tests (8.3, main)	7m 0s
Integration tests (8.2, MOODLE_403_STABLE)	6m 55s
Integration tests (8.2, MOODLE_402_STABLE)	6m 28s
Integration tests (8.1, MOODLE_401_STABLE)	8m 27s
Integration tests (8.0, MOODLE_400_STABLE)	6m 50s
Integration tests (8.0, MOODLE_311_STABLE)	6m 32s
Integration tests (7.4, MOODLE_39_STABLE)	6m 18s
Integration tests (PHAR) (8.3, main)	6m 48s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	8m 0s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 52s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 27s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	6m 24s
	1h 24m 14s

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