Migrate to mill 0.12.5 and refactor mill scripts (#4616) #3250

Job	Run time
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / ci	21m 43s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Formatting	1m 28s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Integration Tests	13m 0s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / compile project with mill	4m 13s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Standard Library (2.13.15)	3m 50s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Build Mdoc & Website	6m 58s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Benchmarking Sanity Check	2m 28s
ci (ubuntu-22.04, 17, 2.13.15, 2.4, 7.0) / Test Scala-CLI Example / Build and Test	2m 39s
check tests	1s
all tests passed	1s
publish	32s
Deploy Website	8s
	57m 1s

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