Milestones to choose from when OTC starts review of Post 3.0.0

[Tomas Mraz]

I've added a few new milestones to choose from during triage when OTC
starts reviewing Post 3.0.0 items in GitHub.

3.0.1 milestone is a milestone for consideration and affirmation if
something is a blocker for the 3.0.1 release. (An issue/PR is a blocker
if 'triaged: OTC evaluated' label is applied.)

3.1.0 milestone is a milestone for consideration and affirmation if
something is a blocker for the 3.1.0 release.

Post 3.1.0 milestone is a milestone for items which we do not want in
3.1.0 but they are not breaking API/ABI so they do not need to be
postponed to the 4.0 release.

-- 
Tomáš Mráz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb
[You'll know whether the road is wrong if you carefully listen to your
conscience.]