Update scala-library to 2.12.20 #2718

Job	Run time
Compile (;testkit/Test/compile;actor-tests/Test/compile)	3m 15s
Compile (;actor-testkit-typed/Test/compile;actor-typed-tests/Test/compile)	3m 12s
Compile (;bench-jmh/Test/compile)	5m 33s
Compile (;cluster/Test/compile;cluster-tools/Test/compile;cluster-typed/Test/compile;distributed-...	7m 22s
Compile (;discovery/Test/compile;coordination/Test/compile)	3m 38s
Compile (;persistence/Test/compile;persistence-shared/Test/compile;persistence-query/Test/compile...	6m 37s
Compile (;pki/Test/compile;slf4j/Test/compile)	2m 54s
Compile (;serialization-jackson/Test/compile)	4m 24s
Compile (;stream/Test/compile;stream-testkit/Test/compile;stream-tests/Test/compile;stream-typed/...	5m 27s
Compile (;stream-tests-tck/Test/compile)	3m 12s
Compile (;remote/Test/compile;remote-tests/Test/compile;protobuf-v3/Test/compile)	4m 57s
	50m 31s

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