Milestone dates should be optional

[carlosmiranda]

Right now milestones.xsd requires the date element. However, the milestone due dates on Github are actually optional:

Let's make the element optional to reflect actual Github rules.

[0crat]

@g4s8/z please, pay attention to this issue

[g4s8]

@0crat in

[0crat]

@0crat in (here)

@g4s8 Job #376 is now in scope, role is DEV

[0crat]

Bug was reported, see §29: +15 point(s) just awarded to @carlosmiranda/z