Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...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>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
         cd build-release
         rm -f -- *.o
         ./prpll -h
-    - uses: actions/upload-artifact@v4
+    - uses: actions/upload-artifact@v6
       if: always()
       with:
         name: ${{ matrix.os }}_${{ matrix.cxx }}_prpll
@@ -79,7 +79,7 @@ jobs:
         cd build-release
         rm *.o
         .\prpll.exe -h
-    - uses: actions/upload-artifact@v4
+    - uses: actions/upload-artifact@v6
       if: always()
       with:
         name: win_${{ matrix.cxx }}_prpll
@@ -100,7 +100,7 @@ jobs:
         cd build-release
         rm -f -- *.o
         ./prpll -h
-    - uses: actions/upload-artifact@v4
+    - uses: actions/upload-artifact@v6
       if: always()
       with:
         name: macos_prpll