chore(deps): bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/firmware-ci.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.ref_type == 'tag'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Check firmware version.txt == tag
         run: |
           # Tag form: vX.Y.Z-esp32  →  expect version.txt to contain X.Y.Z
@@ -70,7 +70,7 @@ jobs:
             artifact_pt: partition-table-c6.bin
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Build firmware (${{ matrix.variant }})
         working-directory: firmware/esp32-csi-node