[Development] Qt 6.0.0 initial schedule

[Jani Heikkinen]

Hi,

It is time to start freezing Qt 6.0.0 initial schedule. I have had some discussion and based on those here is the initial schedule for Qt 6.0.0:

- Qt 6.0 Feature freeze 31.8.2020
- Qt 6.0 Alpha 14.9.2020
- Qt 6.0 Beta 1 1.10..2020
- Qt 6.0.0 RC  17.11.2020
- Qt 6.0.0 Final 1.12.2020

And in addition to these normal release milestones we need to have earlier milestone (Structure and platform freeze) at the end of June (30.6.2020, just before summer holidays) to lock down modules, target platforms etc for the release. In that milestone at least
- New cmake based builds needs to be in place for modules which will be in Qt 6.0.0 release
- All bigger architectural changes needs to be implemented. 
- Target platforms needs to be in CI
(- Something else?)

All that will make sure we can create packaging configurations & start delivering regular snapshots early enough for Qt 6.0.0 (with correct set of modules etc). And this will also ensure all modules can adapt to those big architectural changes before FF. If we don't have this extra milestone we will end up the situation where most of (big) changes will be available just before FF and that will cause huge mess.  

br,
Jani