GitHub Action: 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] <support@github.com>

Author: dependabot[bot]

.github/workflows/prepare-binaries.yml

@@ -77,7 +77,7 @@ jobs:
       run: python scripts/secure_boot_default_keys.py --keystore Templates/LegacyFirmwareDefaults.toml -o FirmwareArtifacts
 
     - name: Upload Firmware Binaries as Artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: Binaries
         path: FirmwareArtifacts/