Consider new sorting schemes for milestones

[bartonjs]

dotnet/machinelearning uses a different numbering scheme than most of the other BCL repositories.

• https://github.com/dotnet/machinelearning/milestones shows an active milestone of "ML.NET 5.0", due Nov 2025.
• https://github.com/dotnet/winforms/milestones shows most milestones don't actually have a due date, so sorting is clearly complicated.
• https://github.com/dotnet/wpf/milestones shows that basically no milestones have a due date.
• https://github.com/dotnet/runtime/milestones shows that all .0 milestones get a due date, others may or may not.

Consider:

• Using dotnet/runtime as the standard bearer, to turn numeric versions into dates.
• Let machinelearning's versions merge in based on their date.
• Happiness ensues?