chore: default deployment target to iOS 15.0, keep 14.0 for release builds

[gmegidish]

Why

Xcode 27 only accepts deployment targets from iOS 15.0, so make sim-install and every other local build failed on main with IPHONEOS_DEPLOYMENT_TARGET is set to 14.0 unless you passed an override by hand.

What

• Project: IPHONEOS_DEPLOYMENT_TARGET goes from 14.0 to 15.0 in all 4 build configurations, so local builds work out of the box.
• Makefile: a new optional IPHONEOS_DEPLOYMENT_TARGET variable. When set, ipa-unsigned, sim-zip-arm64 and sim-zip-x86_64 pass it to xcodebuild; when unset, nothing changes.
• Workflow: the build-ipa and build-sim steps set IPHONEOS_DEPLOYMENT_TARGET: "14.0", so released IPAs and simulator zips still support iOS 14 (chore: restore iOS 14 deployment target #75, fix: mirror swift concurrency runtime into runner below ios 15 #76).
• README: the "Building with Newer Xcode" section now describes this setup instead of the manual override.

Test plan

• make clean && make sim-install on Xcode 27.1 with no override: builds and installs
• make -n: the setting is left out when unset, and passed to all 3 release targets when it is 14.0
• CI: the build-ipa and build-sim jobs succeed with 14.0 on macos-latest

Note: this depends on the Xcode on macos-latest still accepting 14.0. If that runner image moves to Xcode 27, the release builds will need a pinned Xcode or will have to drop iOS 14.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5d72265 and 3e970ba.

📒 Files selected for processing (4)

• .github/workflows/build.yml
• Makefile
• README.md
• devicekit-ios.xcodeproj/project.pbxproj

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[gmegidish]

@hakanor I have changed the default to 15.0 with github building it for 14.0. Otherwise you cannot use local xcode 27.0 to compile this anymore, and it's annoying :).