Bump actions/checkout from 4 to 6 (#4)

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         java: [ "21", "25" ]
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK ${{ matrix.java }}
         uses: actions/setup-java@v4
@@ -55,7 +55,7 @@ jobs:
           - spring-boot: "4.0.4"
             spring: "7.0.6"
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
@@ -76,7 +76,7 @@ jobs:
       matrix:
         kafka: ["3.9.0", "4.0.2"]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v4
         with:
           distribution: 'temurin'