Update fsfe/reuse-action action to v6

Author: renovate[bot]

.github/workflows/main.yml

@@ -55,4 +55,4 @@ jobs:
       - name: Check out the sources
         uses: actions/checkout@v5
       - name: REUSE license check
-        uses: fsfe/reuse-action@v5
+        uses: fsfe/reuse-action@v6