Add Scala 2.12.20 to CI #72

Job	Run time
Integration tests	1m 54s
Build docs	1m 38s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.13)	2m 58s
Check formatting	42s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.14)	2m 55s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.15)	2m 59s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.16)	2m 58s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.17)	2m 57s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.18)	2m 43s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.19)	2m 55s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.20)	2m 49s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.5)	2m 56s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.6)	2m 55s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.7)	2m 56s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.8)	2m 57s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.9)	2m 34s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.10)	2m 51s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.11)	2m 57s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.12)	3m 0s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.13)	2m 54s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.14)	2m 53s
	56m 21s

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