Switch and only maintain mill build

**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`
- [x] `sbt emitVersion` to get version. Switch to use version file.
May solved by #3990
- [x] `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`
- [x] `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
- [x] `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