Add .NET major version release checklist

[lbussell]

Fixes #4574

I will continue to update this as I work on #5005

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[mthalman]

It's great to have this all described in one place.

[MichaelSimons]

I was not expecting this given the title. Should this be in a separate doc?

[lbussell]

I am fine with this going in a separate doc. It's pretty unlikely that we would forget when a major .NET version goes EOL, but I do see value in tracking the date on the board either way.

[mthalman]

IMO, I don't think EOL belongs in a separate doc. I don't see value in that. This is already a long-lived checklist covering alpha to GA so including EOL is just part of the overall lifecycle. The title of the doc could be changed to ".NET Version Lifecyle Checklist" or something.

[MichaelSimons]

If that's the case, this needs rewording File all issues at once when a new .NET version is in Alpha.. IMO we shouldn't be opening an EOL issue until the EOL is known and closer to the EOL date. The presence of this issue sooner could cause confusion.