Update actions/upload-artifact action to v6

Author: renovate[bot]

.github/workflows/docs-pr.yaml

@@ -51,7 +51,7 @@ jobs:
       run: './gradlew --stacktrace asciidoctor javadoc "-Dvariant=5.0" "-DjavaVersion=25"'
     - id: 'step-4'
       name: 'Archive and upload docs'
-      uses: 'actions/upload-artifact@v5'
+      uses: 'actions/upload-artifact@v6'
       with:
         name: 'docs'
         path: |-