Show a warning annotation when "master" branch usage is detected #601

Job	Run time
Code coverage (codecov)	0s
CI test (make validate)	34s
Build moodle-plugin-ci.phar	32s
Integration tests (8.3, main)	6m 30s
Integration tests (8.2, MOODLE_403_STABLE)	6m 3s
Integration tests (8.2, MOODLE_402_STABLE)	5m 58s
Integration tests (8.1, MOODLE_401_STABLE)	6m 50s
Integration tests (8.0, MOODLE_400_STABLE)	6m 57s
Integration tests (8.0, MOODLE_311_STABLE)	6m 12s
Integration tests (7.4, MOODLE_39_STABLE)	5m 56s
Integration tests (PHAR) (8.3, main)	5m 45s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 30s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 40s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 41s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 56s
	1h 15m 4s

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