Update actions/checkout action to v6

.gitea/workflows/ci.yaml

@@ -7,7 +7,7 @@ jobs:
   backend-test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up JDK 25
         uses: actions/setup-java@v4
@@ -21,7 +21,7 @@ jobs:
   frontend-test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Node 24
         uses: actions/setup-node@v4
@@ -51,7 +51,7 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/') && contains(github.ref_name, '.')
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Parse SemVer tag
         id: semver