Switch and only maintain mill build

[Jiuyang Liu]

Type of issue: Feature Request

Is your feature request related to a problem? Please describe.:
The maintenance burden of sbt+sbt is unnecessary, we should kill sbt and switch flow to mill.

Usage in .github

• sbt emitVersion to get version. Switch to use version file.
  May solved by #3990
• sbt fmtCheck to check format.
  use mill __.checkFormat instead, ref to: https://mill-build.com/mill/Scala_Module_Config.html#_reformatting_your_code
  Should be solved by #3992
• sbt $scalaVersion unipublish/mimaReportBinaryIssues
  use mill __.mimaReportBinaryIssues, ref to https://mill-build.com/mill/Thirdparty_Plugins.html#_mima
• sbt $scalaVersion test
• sbt $scalaVersion integrationTests/test
• sbt $scalaVersion standardLibrary/test
  use mill __.test
• sbt ci-release should be replaced by mill cc @jackkoenig)
  May or maynot use https://mill-build.com/mill/Thirdparty_Plugins.html#_ci_release.

Usage in website:

• sbt docs/mdoc
  Ref to https://mill-build.com/mill/Thirdparty_Plugins.html#_mdoc
• sbt emitLatestVersion
  May solved by #3990
• sbt docs/determineContributors
  Add custom task in mill
• sbt docs/generateScalaDocLinks
  Add custom task in mill

We can also create a build target in mill for website