set ios to 14 in github action ci

This fixes the iOS ci which has been broken since https://github.com/google/dawn/actions/runs/19482018618

<details>

<summary> error </summary>

```

/Users/runner/work/dawn/dawn/src/dawn/native/metal/CommandRecordingContext.h:63:41: warning: 'MTLBlitPassDescriptor' is only available on iOS 14.0 or newer [-Wunguarded-availability-new]
   63 |     id<MTLBlitCommandEncoder> BeginBlit(MTLBlitPassDescriptor* descriptor);
      |                                         ^
/Applications/Xcode_26.1.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.1.sdk/System/Library/Frameworks/Metal.framework/Headers/MTLBlitPass.h:65:12: note: 'MTLBlitPassDescriptor' has been marked as being introduced in iOS 14.0 here, but the deployment target is iOS 13.0.0
   65 | @interface MTLBlitPassDescriptor : NSObject <NSCopying>
      |            ^
/Users/runner/work/dawn/dawn/src/dawn/native/metal/CommandRecordingContext.h:63:31: note: annotate 'BeginBlit' with an availability attribute to silence this warning
   63 |     id<MTLBlitCommandEncoder> BeginBlit(MTLBlitPassDescriptor* descriptor);
      |                               ^
/Users/runner/work/dawn/dawn/src/dawn/native/metal/CommandRecordingContext.h:71:47: warning: 'MTLComputePassDescriptor' is only available on iOS 14.0 or newer [-Wunguarded-availability-new]
   71 |     id<MTLComputeCommandEncoder> BeginCompute(MTLComputePassDescriptor* descriptor);
      |                                               ^
/Applications/Xcode_26.1.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.1.sdk/System/Library/Frameworks/Metal.framework/Headers/MTLComputePass.h:67:12: note: 'MTLComputePassDescriptor' has been marked as being introduced in iOS 14.0 here, but the deployment target is iOS 13.0.0
   67 | @interface MTLComputePassDescriptor : NSObject <NSCopying>
      |            ^
/Users/runner/work/dawn/dawn/src/dawn/native/metal/CommandRecordingContext.h:71:34: note: annotate 'BeginCompute' with an availability attribute to silence this warning
   71 |     id<MTLComputeCommandEncoder> BeginCompute(MTLComputePassDescriptor* descriptor);
      |

	  ...

	  /Users/runner/work/dawn/dawn/src/dawn/native/Buffer.cpp:317:24: error: 'notify_all' is unavailable: introduced in iOS 14.0
  317 |         buffer->mState.notify_all();
      |                        ^
/Applications/Xcode_26.1_Release_Candidate.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.1.sdk/usr/include/c++/v1/__atomic/atomic.h:128:56: note: 'notify_all' has been explicitly marked unavailable here
  128 |   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT { std::__atomic_notify_all(*this); }
      |                                                        ^
/Users/runner/work/dawn/dawn/src/dawn/native/Buffer.cpp:853:16: error: 'wait' is unavailable: introduced in iOS 14.0
  853 |         mState.wait(BufferState::PendingMap, std::memory_order_acquire);
      |                ^
/Applications/Xcode_26.1_Release_Candidate.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.1.sdk/usr/include/c++/v1/__atomic/atomic.h:118:3: note: 'wait' has been explicitly marked unavailable here
  118 |   wait(_Tp __v, memory_order __m = memory_order_seq_cst) const _NOEXCEPT {
      |   ^

```

</details>

Ubuntu debug and MacOS x86 debug are also failing sometimes, and aren't fixed here:

ubuntu:

`System.IO.IOException: No space left on device : '/home/runner/actions-runner/cached/_diag/Worker_20251128-093455-utc.log'`

macos x86 debug:

<details>

CMake Error at third_party/CMakeLists.txt:131 (add_subdirectory):
  The source directory

    /Users/runner/work/dawn/dawn/third_party/spirv-headers/src

  does not contain a CMakeLists.txt file.

CMake Error at third_party/CMakeLists.txt:156 (add_subdirectory):
  The source directory

    /Users/runner/work/dawn/dawn/third_party/glslang/src

  does not contain a CMakeLists.txt file.

CMake Error at src/tint/CMakeLists.txt:52 (message):
  Building IR binary format 'TINT_BUILD_IR_BINARY' is enabled, but protobuf
  target is not available, possibly missing
  '/Users/runner/work/dawn/dawn/third_party/protobuf'

</details>

I have not actually tested this because the GitHub free tier seems to limit iOS runners or I ran too many actions today. The test should run on this PR though so we can see if this fixes it.

This is an imported pull request from
https://github.com/google/dawn/pull/51

GITHUB_PR_HEAD_SHA=2b02a212c5347cf5ff90f4bee018308811cf6024
ORIGINAL_AUTHOR=jacksonrl <203005141+jacksonrl@users.noreply.github.com>
GitOrigin-RevId: 953196415a330b99bd4c96d1aa66e2854af319fc
Change-Id: Id332521f03ac7258f2bc62398220e4039ab64232
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/277355
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>