Remove pg12 from github workflow #811

Job	Run time
Build docker image for 13-3.5 variant default	47s
Build docker image for 13-3.5 variant alpine	3m 29s
Build docker image for 14-3.5 variant default	50s
Build docker image for 14-3.5 variant alpine	3m 16s
Build docker image for 15-3.5 variant default	49s
Build docker image for 15-3.5 variant alpine	3m 18s
Build docker image for 16-3.5 variant default	50s
Build docker image for 16-3.5 variant alpine	3m 20s
Build docker image for 17-3.5 variant default	44s
Build docker image for 17-3.5 variant alpine	3m 16s
Build docker image for 16-master variant default	33m 20s
Build docker image for 17-master variant default	32m 46s
	1h 26m 45s

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