Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
         runs-on: ubuntu-latest
 
         steps:
-            - uses: actions/checkout@v5
+            - uses: actions/checkout@v6
 
             - uses: shivammathur/setup-php@v2
               with:
@@ -40,7 +40,7 @@ jobs:
                       composer-flags: '--prefer-lowest'
 
         steps:
-            - uses: actions/checkout@v5
+            - uses: actions/checkout@v6
 
             - uses: shivammathur/setup-php@v2
               with:
@@ -72,7 +72,7 @@ jobs:
         runs-on: ubuntu-latest
 
         steps:
-            - uses: actions/checkout@v5
+            - uses: actions/checkout@v6
 
             - uses: shivammathur/setup-php@v2
               with:
@@ -90,7 +90,7 @@ jobs:
         runs-on: ubuntu-latest
 
         steps:
-            - uses: actions/checkout@v5
+            - uses: actions/checkout@v6
 
             - uses: shivammathur/setup-php@v2
               with: