Bump mill to 0.12

[Gedochao]

Migrate to 0.12, they said.
It will be fun, they said.

Special thanks to @lolgab, who trailblazed this back in #3250

What this does:

• bumps mill to 0.12.14
• migrates build configuration files to .mill.scala, along with the new syntax
• bumps mill-scalafix to 0.5.1
  • a new, dedicated scalafix configuration has been added specifically for modules which are cross compiled against Scala 2.12, 2.13 and 3 (such as runner or test-runner)
  • RemoveUnused has been enabled for Scala 3
    • a side effect is that I had to do some funny workarounds with imports because of an obscure linting bug that's present in Scala 3.3.6, but not in 3.7 (and we do cross-compile with scalafix)
    • just the 0.5.1 introduced a load of changes, and bringing the configuration consistent for all cross-compiled options was funny, the steps are in separate commits
• bumps mill-native-image and mill-native-image-upload to 0.1.31-1
• a myriad of smaller things, hacks and workarounds

[lolgab]

You can add me as co-author in the final merge with the following:

Co-authored-by: Lorenzo Gabriele <[email protected]>

[Gedochao]

Rebased on top of main to solve conflicts + cleaned the commits structure a bit.

[tgodzik]

Wow, that was a lot of changes! Great work! I think it's mostly done, I just reported some smaller issues and have some questions

[tgodzik]

LGTM only some minor comments for future consideration. Great work!