macOS: cannot configure with Xcode 27 — CMAKE_OSX_DEPLOYMENT_TARGET pinned to 11.3, but Xcode 27 requires >= 12.0

Problem

On Xcode 27, configuring OrcaSlicer fails immediately at project() with the Xcode generator — before any source file is compiled:

CompilerIdC.xcodeproj: error: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 11.3,
but the range of supported deployment target versions is 12.0 to 27.0.x.
(in target 'CompilerIdC' from project 'CompilerIdC')

which makes CMake's compiler identification fail, and configuration aborts with:

-- The C compiler identification is unknown
-- The CXX compiler identification is unknown
CMake Error at CMakeLists.txt:73 (project):
  No CMAKE_C_COMPILER could be found.

Cause

CMakeLists.txt defaults the deployment target to 11.3:

https://github.com/OrcaSlicer/OrcaSlicer/blob/main/CMakeLists.txt#L59-L62

Xcode 27's build system rejects any deployment target below 12.0, so the compiler-identification sub-project cannot build.

Worth noting: raw clang still accepts -mmacosx-version-min=11.3 without complaint — it is xcodebuild that enforces the 12.0 floor. So this affects the Xcode generator specifically, which is what build_release_macos.sh selects by default (SLICER_CMAKE_GENERATOR="Xcode").

Steps to reproduce

Clean checkout of main (c833ccdf), empty build directory:

cmake -S . -B build/arm64 -G Xcode -DCMAKE_OSX_ARCHITECTURES=arm64

Environment:

• Xcode 27.0 (build 27A266a), macOS 26 SDK → MacOSX27.0.sdk
• Apple clang 21.0.0 (clang-2100.3.34.2)
• CMake 3.31.4
• Apple Silicon (arm64)

Passing -DCMAKE_OSX_DEPLOYMENT_TARGET=12.0 gets past it, and configuration then proceeds normally.

Suggested fix

Either raise the default floor to 12.0, or clamp the default to the lowest deployment target the active toolchain still supports, so older Xcode versions keep their current behaviour.

Happy to open a PR — just say which direction you'd prefer (flat bump vs. version-aware clamp).