CI: Update steam-upload runner to macos-15

.github/workflows/publish.yaml

@@ -161,7 +161,7 @@ jobs:
     name: Upload Steam Builds 🚂
     needs: check-tag
     if: github.repository_owner == 'obsproject' && fromJSON(needs.check-tag.outputs.validTag)
-    runs-on: macos-14
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/steam-upload