Raise deployment targets so the project builds on Xcode 27

[MansiVisuals]

Xcode 27 will not build the project as it stands. Both schemes fail before
compiling anything:

error: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to
10.13.6, but the range of supported deployment target versions is 12.0 to 27.0.x.
(in target 'mft' from project 'mft')

error: The iOS deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 13.5,
but the range of supported deployment target versions is 15.0 to 27.0.x.
(in target 'mft ios' from project 'mft')

This raises the minimums to the lowest versions Xcode 27 still accepts, 12.0
for macOS and 15.0 for iOS. That is the whole change, six lines in
project.pbxproj. Both schemes then build clean with no source changes: the
limit is what the toolchain is willing to emit for, not anything the code
actually does.

Worth being explicit that this is a breaking change for anyone embedding the
framework who still targets macOS 10.13 to 11 or iOS 13.5 to 14. Xcode 26.6
builds the current settings fine, so this only bites once you move to 27, and
it is entirely your call whether dropping those versions is the trade you want
to make now. Happy to close this if you would rather hold the wider support and
stay on 26.6 for the time being.

Verified on Xcode 27.0 (27A266a): xcodebuild -scheme mft -destination 'platform=macOS' and xcodebuild -scheme 'mft ios' -destination 'generic/platform=iOS' both report BUILD SUCCEEDED, with no command line
overrides.

[mplpl]

Yes, that is unfortunate but true. I also expect that in a few months apps built using Xcode 26 will no longer be accepted in AppStore. After that, use of Xcode 27 will become mandatory and support for macOS prior to version 12 will have to be dropped anyway.