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/assembly.yml

@@ -19,27 +19,27 @@ jobs:
 
       - name: robocode.core test artifacts
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: robocode.core-test-result
           path: robocode.core/build/reports/tests/test
 
       - name: robocode.host test artifacts
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: robocode.host-test-result
           path: robocode.host/build/reports/tests/test
 
       - name: robocode.tests test artifacts
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: robocode.tests-result
           path: robocode.tests/build/reports/tests/test
 
       - name: setup binary artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: robocode-setup
           path: build/robocode-*-setup.jar