GH Actions: bump actions/checkout to v6 (was v5)

See https://github.com/actions/checkout/blob/0c366fd6a839edf440554fa01a7085ccba70ac98/README.md#checkout-v6

Author: generalmimon

.github/workflows/release.yml

@@ -15,7 +15,7 @@ jobs:
     permissions:
       contents: read
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Set up Python