Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     timeout-minutes: 10
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: pnpm/action-setup@v4
         with:
           version: 10
@@ -44,7 +44,7 @@ jobs:
     timeout-minutes: 10
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: pnpm/action-setup@v4
         with:
           version: 10
@@ -79,7 +79,7 @@ jobs:
           - ember-cp-validations-6
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: pnpm/action-setup@v4
         with:
           version: 10