Avoid spurious deprecation warnings in generated code (#3379)

Fixes #3371

The direct problem is that we are generating code with resolved type
aliases, resulting in the `JavaTests` aliases being resolved to
deprecated `JavaModuleTests` traits. However, in general generated code
should probably not display warnings at all, so better to just slap
`@nowarn` on the whole chunk rather than trying to fix the immediate
alias resolution problem

runner/src/mill/runner/MillBuildRootModule.scala

@@ -368,6 +368,7 @@ object MillBuildRootModule {
          |import _root_.mill.runner.MillBuildRootModule
          |$imports
          |
+         |@scala.annotation.nowarn
          |object ${backtickWrap(miscInfoName)} {
          |  implicit lazy val millBuildRootModuleInfo: _root_.mill.runner.MillBuildRootModule.Info = _root_.mill.runner.MillBuildRootModule.Info(
          |    ${enclosingClasspath.map(p => literalize(p.toString))}.map(_root_.os.Path(_)),