Migrate integration tests to Scala 3

[Gedochao]

What was done

• the integration module is now fully built with Scala 3.3.7
• scala-async syntax has been rewritten with vanilla Scala 3 Futures syntax
• a number of minor workarounds were necessary because of issues with the expecty dependency
  • expect crashes at runtime when called with a Java enum eed3si9n/expecty#212 / os-lib 0.11.4 fails at runtime with expecty com-lihaoyi/os-lib#361
  • os-lib 0.11.4 fails at runtime with expecty eed3si9n/expecty#197 / [Scala 3] Macro leads to class-not-found at runtime when containing a reference to a java enum.  eed3si9n/expecty#59

Potential follow-ups

• completely remove expecty from the project
  • way out of scope for this PR
  • this was tempting to do here, but requires a project-wide refactor
  • tl;dr we now have the workarounds around the issues mentioned above everywhere, and the library is perhaps more trouble than it is worth
• cross-compile the integration module against Scala 3 Next versions
  • this would be a good idea both for dogfooding and in preparation to the 3.9 LTS bump
  • it may be a good idea to finally bump to Mill 1.x, first

[zielinsky]

LGTM! Just one minor change from me