Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/tts-intelligibility.yml

@@ -51,7 +51,7 @@ jobs:
           command -v mimic
 
       - name: Cache models (huggingface + plugin data)
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: |
             ~/.cache/huggingface