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](actions/upload-artifact@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>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -71,7 +71,7 @@ jobs:
         python runTests.py -j4 ${{ matrix.config.tests }}
 
     - name: Upload gtest_output xml
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       if: failure()
       with:
         name: gtest_outputs_xml
@@ -134,7 +134,7 @@ jobs:
         python runTests.py -j2 $MixFunProbTestsArray[(${{ matrix.group }} - 1)]
 
     - name: Upload gtest_output xml
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       if: failure()
       with:
         name: gtest_outputs_xml