CI: Increase macOS deployment target to 10.15 for x86_64

.github/workflows/main.yml

@@ -73,7 +73,7 @@ jobs:
     if: always()
     needs: [clang_check]
     env:
-      MACOSX_DEPLOYMENT_TARGET_X86_64: '10.13'
+      MACOSX_DEPLOYMENT_TARGET_X86_64: '10.15'
       MACOSX_DEPLOYMENT_TARGET_ARM64: '11.0'
       SPARKLE_VERSION: '1.26.0'
       SPARKLE_HASH: '8312cbf7528297a49f1b97692c33cb8d33254c396dc51be394e9484e4b6833a0'