update the "checkout" action to v6

Author: stephengold

.github/workflows/push.yml

@@ -9,7 +9,7 @@ jobs:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v5
         with:
           distribution: 'zulu'
@@ -25,7 +25,7 @@ jobs:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
     runs-on: windows-2025
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v5
         with:
           distribution: 'liberica'