iOS: Update deployment target to iOS 14

[oskirby]

Description

Since Qt 6.x, the minimum supported OS version for iOS is 14.0. However, we have been setting the CMAKE_OSX_DEPLOYMENT_TARGET to 13.0. This results in an overwhelming flood of linker warnings about deployment target mismatches. We should update this to match the version requirements of Qt.

Reference

Qt iOS documentation: Qt for iOS UNNotificationPresentationOptionAlert: deprecation notice

VPN-6385

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

[lesleyjanenorton]

We have confirmation from @albionx that this is ok (https://mozilla-hub.atlassian.net/browse/VPN-5561). Anything else we are waiting on here?

[lesleyjanenorton]

Updated description to ref VPN-6385.

[strseb]

https://github.com/mozilla-mobile/mozilla-vpn-client/pull/9677 <- This superseeds this i think :)