Lower macOS build runner to macOS 15 with Xcode 16.4

Changed mac_notarized job to use:
- runs-on: macos-15 (was macos-latest)
- xcode-version: 16.4 (was 26.0.1)

This configuration was previously working for Yattee builds.
iOS and tvOS builds remain on macos-latest with Xcode 26.0.1.

Reference: https://github.com/actions/runner-images/blob/main/images/macos/macos-15-arm64-Readme.md

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/release.yml

@@ -52,7 +52,7 @@ jobs:
           if-no-files-found: ignore
   mac_notarized:
     name: Build and notarize macOS app
-    runs-on: macos-latest
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
@@ -66,7 +66,7 @@ jobs:
           sed -i '' 's/3rd Party Mac Developer Application/Developer ID Application/' Yattee.xcodeproj/project.pbxproj
       - uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: '26.0.1'
+          xcode-version: '16.4'
       - uses: maierj/fastlane-action@v3.0.0
         with:
           lane: mac build_and_notarize