[Feature Request] Create `vNext` milestone

[filzrev]

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)