Bump actions/checkout from 4 to 6 (#10)

Author: dependabot[bot]

.github/workflows/release-plz.yml

@@ -19,7 +19,7 @@ jobs:
     environment: Cargo
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: initial submodule