Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci-alpine.yml

@@ -14,7 +14,7 @@ jobs:
       CXX: ${{ matrix.compiler }}
       CXXFLAGS: -Werror
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install dependencies
         run: |

.github/workflows/ci-linux.yml

@@ -13,7 +13,7 @@ jobs:
       CXX: ${{ matrix.compiler }}
       CXXFLAGS: -Werror
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install dependencies
         run: |

.github/workflows/ci-windows.yml

@@ -17,7 +17,7 @@ jobs:
       CONAN_USER_HOME: ${{ github.workspace }}/win/build
       CONAN_USER_HOME_SHORT: None
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Setup NSIS
         run: |-