ci: 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/release-spectre-demo.yml

@@ -101,7 +101,7 @@ jobs:
           tar -czvf ../${{ matrix.artifact }}.tar.gz *
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ matrix.artifact }}
           path: archive/${{ matrix.artifact }}.*

.github/workflows/release.yml

@@ -99,7 +99,7 @@ jobs:
           tar -czvf ../${{ matrix.artifact }}.tar.gz *
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ matrix.artifact }}
           path: archive/${{ matrix.artifact }}.*