Downgrade Rebar3 to 3.20.0 for OTP 22/23 #3

Job	Run time
docker (DIR=master)	9m 45s
docker (DIR=master VARIANT=alpine)	7m 20s
docker (DIR=27)	8m 32s
docker (DIR=27 VARIANT=slim)	7m 45s
docker (DIR=27 VARIANT=alpine)	8m 8s
docker (DIR=26)	13m 20s
docker (DIR=26 VARIANT=slim)	7m 31s
docker (DIR=26 VARIANT=alpine)	7m 46s
docker (DIR=25)	11m 54s
docker (DIR=25 VARIANT=slim)	6m 23s
docker (DIR=25 VARIANT=alpine)	6m 57s
docker (DIR=24)	9m 59s
docker (DIR=24 VARIANT=slim)	6m 25s
docker (DIR=24 VARIANT=alpine)	7m 0s
docker (DIR=23)	10m 35s
docker (DIR=23 VARIANT=slim)	6m 45s
docker (DIR=23 VARIANT=alpine)	6m 49s
docker (DIR=22)	6m 53s
docker (DIR=22 VARIANT=slim)	6m 18s
docker (DIR=22 VARIANT=alpine)	6m 17s
docker (DIR=21)	7m 15s
docker (DIR=21 VARIANT=slim)	5m 30s
docker (DIR=20)	6m 23s
docker (DIR=20 VARIANT=slim)	5m 3s
	3h 6m 33s

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