migrate to CI3 plugin system. Closes: #518 #849

Job	Run time
Get PHP versions to test	4s
check-code (ubuntu-latest, 8.2)	1m 59s
check-translation (ubuntu-latest, 7.2)	24s
test (ubuntu-latest, 8.0)	1m 7s
test (ubuntu-latest, 8.1)	50s
test (ubuntu-latest, 8.2)	47s
test (ubuntu-latest, 8.3)	48s
test (ubuntu-latest, 7.0)	1m 14s
test (ubuntu-latest, 7.1)	54s
test (ubuntu-latest, 7.2)	56s
test (ubuntu-latest, 7.3)	1m 4s
test (ubuntu-latest, 7.4)	1m 3s
test (ubuntu-latest, 5.6)	1m 2s
	12m 12s

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