Move away from using sbt integration config

[mdedetrich]

See https://github.com/sbt/sbt/releases/tag/v1.9.0-RC3, specifically

Deprecation of IntegrationTest configuration
sbt 1.9.0 deprecates IntegrationTest configuration. (RFC-3 proposes to deprecate general use of configuration axis beyond Compile and Test, and this is the first installment of the change.)

The recommended migration path is to create a subproject named "integration", or "foo-integration" etc.

lazy val integration = (project in file("integration"))
  .dependsOn(core) // your current subproject
  .settings(
    publish / skip := true,
    // test dependencies
    libraryDependencies += something % Test,
  )