[TIMOB-25202] Windows: Drop support for targeting Windows Phone 8.1 and Windows 8.1

Description

This epic is to track the different aspects of removing Windows Phone 8.1 and Windows 8.1 from Titanium SDK and any related tooling libraries. At a pretty high level, not in a specific order: * Stop producing and testing in CI the relevant build artifacts * Remove platform specific code from within the Titanium Windows source files * Update documentation * Remove support from various scripts around building the SDK and stuff * Spit out a warning? (Probably needs to be slid in before 7, maybe in 6.3.0 or 6.4.0) * After this is done, close any outstanding tickets as wont fix * Before doing this, identify if there are any high profile tickets we wish to fix before dropping * Have Studio remove the different 8.1 related views (packaging dialog with the SDK choice, not sure on others). Will file the ticket once this is all approved This *has to be done* for the 7.0.0 timeframe, keeping 8.1 around for another year (we plan 1 major rev a year as I understand) would be a bad choice that would severely limit our ability to move things forward and keep progressing the Windows Platform at the same rate as iOS and Android. I don't believe our current deprecation policy reserves our right to remove a platform at any time like the currently proposed update. Extra supporting bits My prior writing on why we should do this https://wiki.appcelerator.org/pages/viewpage.action?pageId=50168498 Article regarding Microsoft dropping Windows Phone 8.1 [Goodbye Windows Phone: What comes next for Microsoft in mobile?](http://www.zdnet.com/article/goodbye-windows-phone-what-comes-next-for-microsoft-in-mobile) Or if you prefer a nice clickbaity headline [Windows Phone dies today](https://www.theverge.com/2017/7/11/15952654/microsoft-windows-phone-end-of-support)