Show a warning annotation when "master" branch usage is detected #581

Job	Run time
Code coverage (codecov)	37s
CI test (make validate)	51s
Build moodle-plugin-ci.phar	42s
Integration tests (8.3, main)	7m 4s
Integration tests (8.2, MOODLE_403_STABLE)	6m 29s
Integration tests (8.2, MOODLE_402_STABLE)	6m 16s
Integration tests (8.1, MOODLE_401_STABLE)	7m 24s
Integration tests (8.0, MOODLE_400_STABLE)	6m 37s
Integration tests (8.0, MOODLE_311_STABLE)	6m 29s
Integration tests (7.4, MOODLE_39_STABLE)	5m 54s
Integration tests (PHAR) (8.3, main)	6m 10s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	6m 33s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	6m 19s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	7m 9s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 57s
	1h 20m 31s

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