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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -44,7 +44,7 @@ jobs:
         run: dotnet build --configuration Release --no-restore
 
       - name: Upload build artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: build-artifacts
           path: |
@@ -85,7 +85,7 @@ jobs:
             --results-directory ./TestResults
 
       - name: Upload test results
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         if: always()
         with:
           name: test-results
@@ -158,14 +158,14 @@ jobs:
             -p:SymbolPackageFormat=snupkg
 
       - name: Upload package artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: nuget-packages
           path: ./artifacts/*.nupkg
           retention-days: 30
 
       - name: Upload symbol packages
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: nuget-symbols
           path: ./artifacts/*.snupkg