Add Scala 2.12.20 support #213

Job	Run time
Integration tests	2m 5s
Build docs	1m 48s
Check formatting	53s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.13)	2m 56s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.14)	2m 58s
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 54s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.18)	2m 59s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.19)	3m 2s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.20)	2m 48s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.5)	3m 2s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.6)	3m 1s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.7)	3m 4s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.8)	2m 59s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.9)	2m 53s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.10)	2m 53s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.11)	3m 4s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.12)	2m 59s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.13)	2m 52s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.14)	2m 53s
	58m 0s

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