Update actions/checkout action to v6

| datasource  | package          | from | to |
| ----------- | ---------------- | ---- | -- |
| github-tags | actions/checkout | v5   | v6 |

Author: renovate[bot]

.github/workflows/backwards-compatibility.yml

@@ -11,7 +11,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
         with:
           fetch-depth: 0
 

.github/workflows/coding-standards.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/[email protected]"

.github/workflows/composer-json-lint.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/setup-php@v2"

.github/workflows/mutation-tests.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/[email protected]"

.github/workflows/phpunit.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/[email protected]"
@@ -81,7 +81,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/[email protected]"

.github/workflows/release-on-milestone-closed.yml

@@ -14,7 +14,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Release"
         uses: "laminas/[email protected]"

.github/workflows/static-analysis.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
 
       - name: "Install PHP"
         uses: "shivammathur/[email protected]"