chore(ci): 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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build-firmware.yml

@@ -12,6 +12,6 @@ jobs:
         run: cd /usr/src/ && ./build.sh || cat /usr/src/log.txt
 
       - name: Archive produced firmware
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           path: /usr/src/out/open_source/open_source.bin