Address review: split macOS Swift checks by CodeQL version

Rather than pinning every macOS job to macOS 15, only run the older CodeQL CLI
versions (which need Xcode 16, and so macOS 15) there. Newer versions, which
support Swift 6.2, run on the latest macOS runner with its default Xcode, so we
keep exercising the common latest-CLI-on-latest-runner combination.

To express this, `sync.ts` now supports an optional per-OS-entry
`codeql-versions` filter, used by `multi-language-autodetect` to send old
versions to `macos-15-xlarge` and new versions to `macos-latest-xlarge`.

`swift-custom-build` only runs `linked`/`default`/`nightly`, which all support
Swift 6.2, so it no longer needs the Xcode 16 step or a macOS 15 pin.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/__multi-language-autodetect.yml

@@ -81,19 +81,19 @@ jobs:
             version: stable-v2.23.9
           - os: ubuntu-latest
             version: stable-v2.24.3
-          - os: macos-15-xlarge
+          - os: macos-latest-xlarge
             version: stable-v2.24.3
           - os: ubuntu-latest
             version: default
-          - os: macos-15-xlarge
+          - os: macos-latest-xlarge
             version: default
           - os: ubuntu-latest
             version: linked
-          - os: macos-15-xlarge
+          - os: macos-latest-xlarge
             version: linked
           - os: ubuntu-latest
             version: nightly-latest
-          - os: macos-15-xlarge
+          - os: macos-latest-xlarge
             version: nightly-latest
     name: Multi-language repository
     if: github.triggering_actor != 'dependabot[bot]'
@@ -130,7 +130,8 @@ jobs:
           python-version: '3.13'
 
       - name: Use Xcode 16
-        if: runner.os == 'macOS' && matrix.version != 'nightly-latest'
+        # Only the older CodeQL CLI versions need Xcode 16, and these run on macOS 15.
+        if: matrix.os == 'macos-15-xlarge'
         run: sudo xcode-select -s "/Applications/Xcode_16.app"
 
       - uses: ./../action/init

.github/workflows/__swift-custom-build.yml

@@ -59,11 +59,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: macos-15
+          - os: macos-latest
             version: linked
-          - os: macos-15
+          - os: macos-latest
             version: default
-          - os: macos-15
+          - os: macos-latest
             version: nightly-latest
     name: Swift analysis using a custom build command
     if: github.triggering_actor != 'dependabot[bot]'
@@ -91,9 +91,6 @@ jobs:
           version: ${{ matrix.version }}
           use-all-platform-bundle: 'false'
           setup-kotlin: 'true'
-      - name: Use Xcode 16
-        if: runner.os == 'macOS' && matrix.version != 'nightly-latest'
-        run: sudo xcode-select -s "/Applications/Xcode_16.app"
       - uses: ./../action/init
         id: init
         with: