Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/check.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           token: ${{ secrets.PAT }}
       - name: Get the latest version of OpenVPN

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
       BUILD_ID: ${{ github.run_number }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Export OpenVPN version to variable
         run: echo "OPENVPN_VERSION=$(cat version)" >> $GITHUB_ENV