Switch build to Mill

[alexarchambault]

Currently having issues with publishing on the CI. sbt-ci-release and sbt-pgp don't really allow for fine grained customization of gpg-related things (passing custom gpg options, etc.). We should switch the build here to Mill, and use the same setup as many of my other projects (setup that must itself be based on the Ammonite's one IIRC), not using mill-ci-release, that doesn't allow for customization / tweakings of gpg things either. This particular setup, with secrets uploaded via touch Foo.scala && scala-cli publish setup --ci Foo.scala has never failed me.