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/documentation.yml

@@ -51,7 +51,7 @@ jobs:
         run: ./gradlew generateDocs --stacktrace
 
       - name: Upload generated documentation
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: operator-documentation
           path: |
@@ -61,7 +61,7 @@ jobs:
 
       - name: Upload documentation preview (PR only)
         if: github.event_name == 'pull_request'
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: documentation-preview-${{ github.event.number }}
           path: |

.github/workflows/schema-validation.yml

@@ -44,7 +44,7 @@ jobs:
 
     - name: Upload validation artifacts
       if: failure()
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: validation-logs
         path: |