Update zinc to 1.6.1

[scala-steward]

Updates org.scala-sbt:zinc from 1.5.9 to 1.6.1.
GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

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labels: library-update, early-semver-minor, semver-spec-minor, commit-count:1

[lefou]

After re-sync with merged #1713 , this PR should build successfully.

[lolgab]

@lefou This could be a good opportunity to remove the forced log4j core dependency since Zinc 1.6.1 already depends on 2.17.1

[lefou]

@lefou This could be a good opportunity to remove the forced log4j core dependency since Zinc 1.6.1 already depends on 2.17.1

I'm a huge fan of explicit dependencies. Hence I'd like to keep it, especially now, as many people are aware of issues with Log4j2 and are seeking for the concrete version Mill might use. Unless zinc drops it, it doesn't hurt to keep it declared. If next zinc pulls a newer version, we'll get it automatically. If they don't, we or probably scala-steward will bump it and we are also good to go.