Bump actions/upload-artifact from 5 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build-libs.yml

@@ -37,7 +37,7 @@ jobs:
           file libportaudio.dylib
           otool -L libportaudio.dylib
       - name: upload dylib
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: macos-dylib
           path: libportaudio.dylib
@@ -85,7 +85,7 @@ jobs:
         run: |
           ldd libportaudio${{ matrix.platform-suffix }}${{ matrix.asio }}.dll
       - name: upload DLL
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: windows-${{ matrix.platform }}-dll${{ matrix.asio }}
           path: libportaudio${{ matrix.platform-suffix }}${{ matrix.asio }}.dll