chore(deps): 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](https://github.com/actions/upload-artifact/compare/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] <support@github.com>

.github/workflows/classic-autogpt-ci.yml

@@ -139,7 +139,7 @@ jobs:
 
       - name: Upload logs to artifact
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: test-logs
           path: classic/original_autogpt/logs/

.github/workflows/classic-forge-ci.yml

@@ -237,7 +237,7 @@ jobs:
 
       - name: Upload logs to artifact
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: test-logs
           path: classic/forge/logs/

.github/workflows/platform-frontend-ci.yml

@@ -269,7 +269,7 @@ jobs:
 
       - name: Upload Playwright report
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: playwright-report
           path: playwright-report
@@ -278,7 +278,7 @@ jobs:
 
       - name: Upload Playwright test results
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: playwright-test-results
           path: test-results