Bump actions/download-artifact from 5 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v5...v6)

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

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

.github/workflows/ci_build_major_branch.yml

@@ -87,7 +87,7 @@ jobs:
           fetch-depth: 0
 
       - name: Download firmwares
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           pattern: firmware-*
           path: .

.github/workflows/ci_build_major_branch_keymap.yml

@@ -92,7 +92,7 @@ jobs:
         uses: actions/checkout@v5
 
       - name: Get target definitions
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: targets-${{ inputs.keymap }}
           path: .
@@ -139,7 +139,7 @@ jobs:
         uses: actions/checkout@v5
 
       - name: Download firmwares
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           pattern: firmware-${{ inputs.keymap }}-*
           path: .