build: remove hardcoded Scala 2.12 version from gen-scala-server scripted tests; add scala-2_13 test suite #1609

Job	Run time
Gradle tests	2m 53s
Checks	1m 13s
Docs	2m 46s
Compile and test (3.3.7, 21)	3m 56s
sbt scripted tests (scala3, 3.3)	3m 48s
sbt scripted tests (gen-scala-server, 2.13)	7m 39s
Compile and test (3.3.7, 17)	4m 32s
Maven tests	2m 26s
Compile and test (2.13.18, 17)	4m 14s
Compile and test (2.13.18, 21)	3m 42s
sbt scripted tests (gen-java, 2.13)	4m 15s
Compile Benchmarks	1m 40s
	43m 4s

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