ci: Bump macos runner images (#417)

Closes #416.

Workflows using x86_64-apple-darwin target with a `macos-13` runner
image have been migrated to `macos-15-intel` due to deprecation (see
#416), and `macos-14` images have been migrated to `macos-15`.

.github/workflows/test-llvm-builder.yml

@@ -18,7 +18,7 @@ jobs:
   test:
     strategy:
       matrix:
-        runner: [parity-large, macos-14, windows-2022]
+        runner: [parity-large, macos-15, windows-2022]
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
@@ -36,7 +36,7 @@ jobs:
           sudo apt update && sudo apt-get install -y cmake ninja-build curl git libssl-dev pkg-config clang lld musl
 
       - name: Install Dependencies
-        if: matrix.runner == 'macos-14'
+        if: matrix.runner == 'macos-15'
         run: |
           brew install ninja