Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/publish.yml

@@ -12,7 +12,7 @@ jobs:
     if: github.repository_owner == 'statnett'
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           fetch-depth: 0
       - name: Install the latest version of uv and set the python version

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13"]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           fetch-depth: 0
       - name: Install the latest version of uv and set the python version
@@ -26,7 +26,7 @@ jobs:
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13"]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           fetch-depth: 0
       - name: Install the latest version of uv and set the python version