use versionfile for mill

[sequencer]

Reference: https://mill-build.com/mill/contrib/versionfile.html

This provide a unified version management target for future releasing.

After switch to mill, I'd like to store all dependency versions to version folder for reference, CI bump and review.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Feature (or new API)
• API modification
• API deprecation
• Backend code generation
• Performance improvement
• Bugfix
• Documentation or website-related
• Dependency update
• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).
• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[jackkoenig]

With the current SBT flow, every push to main publishes a SNAPSHOT based on the current commit. That wouldn't work with a version file.

Similarly, the current flow for tagged releases uses the git tag as the version. We could use a version file but that would require making an extra commit for every release that isn't needed in the current flow.

[sequencer]

With the current SBT flow, every push to main publishes a SNAPSHOT based on the current commit. That wouldn't work with a version file.
Similarly, the current flow for tagged releases uses the git tag as the version. We could use a version file but that would require making an extra commit for every release that isn't needed in the current flow.

Yes, we should dig out a way to maintain version in mill, this is just an example, we can override functions in the VersionFileModule trait, or even get rid of it. This is just for discussion.