Add partial cross-compilation for Scala 3 #3065

Job	Run time
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / compile project with mill	5m 12s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / ci	10m 25s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Benchmarking Sanity Check	2m 47s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Formatting	1m 33s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Integration Tests	12m 39s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Standard Library (2.13.15)	4m 44s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Build Mdoc & Website	7m 33s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Test Scala-CLI Example / Build and Test	3m 4s
check tests	0s
all tests passed	1s
Deploy Website	0s
publish	0s
	47m 58s

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