chore(deps): update actions/upload-artifact action to v6

Author: renovate[bot]

.github/workflows/gradle.yml

@@ -24,7 +24,7 @@ jobs:
         max_attempts: 2
         command: ./gradlew clean build
     - name: Upload build artifact
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: SignedVelocity
         path: build/SignedVelocity-*.jar