removed macos12 from CI

We use kiwix-build deps which are compiled with (and instruct to build with) Xcode 15
which is not available on macOS12
https://github.com/actions/runner-images/blob/main/images/macos/macos-12-Readme.md#xcode

Also removed macos13-xlarge (apple silicon) as it's completly excluded ; so it's simpler

.github/workflows/ci.yml

@@ -12,21 +12,11 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - macos-12
           - macos-13
-          - macos-13-xlarge
         target:
           - native_dyn
           - iOS_arm64
           - iOS_x86_64
-        exclude:
-          # iOS cross-compilation only on the 2 latest versions of macOS x86_64
-          - os: macos-13-xlarge
-            target: iOS_arm64
-          - os: macos-13-xlarge
-            target: iOS_x86_64
-          # TODO: M1 runner is still in beta and not reliable
-          - os: macos-13-xlarge
     runs-on: ${{ matrix.os }}
 
     steps: