VPN-6742: update minimum iOS version to iOS 15

[mcleinman]

Description

Change minimum version, remove unneeded code.

Reference

VPN-6742

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[mcleinman]

I'm having trouble getting a local iOS build going. However, if this builds on the server, we should be good to go. (I recently updated macOS, which required switching to a new version of Xcode. I've solved 3 different blockers to getting a build going, am still running into other novel issues.)

[oskirby]

That turned out to be a very messy diff for just deleting code, but I think it's good.