[Feature Request] Create `vNext` milestone

**Is your feature request related to a problem? Please describe.**

Currently, docfx using `Backlog` and `Working Set` milestones..
But it seems not be maintained actively.

https://github.com/dotnet/docfx/milestones

**Describe the solution you'd like**
Create `vNext` milestone. and run following steps when new docfx version is released (on `release.yml`).
1, Rename existing `vNext` milestone to `v2.x.x`.
2. Create new `vNext` milestone

**Additional context**
By this changes.
It can find what's docfx version resolve specific issues.

And it can keep issue opened until corresponding docfx version released.
(Currently issue is closed when PR is merged. but it's not friendly for user)