chore(deps): update actions/cache action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
           python-version: "3.14"
 
       - id: cache
-        uses: actions/cache@9255dc7a253b0ccc959486e2bca901246202afeb # v5
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt', '**/test_requirements.txt') }}