Update cordova-ios to 6.x

[jthrilly]

Updating cordova IOS would allow us to keep up with bugfixes and improvements.

Changes needed:

• [ ] Remove wkwebview-engine - no longer needed, since it is now integrated at the platform level
• [ ] Refactor to remove FileTransfer plugin, since it is not compatible. Will require new code to handle filewriting: https://cordova.apache.org/blog/2017/10/18/from-filetransfer-to-xhr2.html

Alternative:

An alternative to this is to explore the idea of converting to capacitor: https://capacitorjs.com/