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](actions/checkout@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] <[email protected]>

Author: dependabot[bot]

.github/workflows/coverage.yml

@@ -18,7 +18,7 @@ jobs:
         os: [ubuntu-latest]
         python-version: ["3.13"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - name: Set up Python ${{ matrix.python-version }}

.github/workflows/lint.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
           fetch-depth: 0

.github/workflows/test_development_versions.yml

@@ -30,7 +30,7 @@ jobs:
             compiler: g++
             cxx_standard: 23
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - name: Check out default branch of each submodule

.github/workflows/test_latest_versions.yml

@@ -47,7 +47,7 @@ jobs:
           - os: ubuntu-22.04-arm
             compiler: clang++
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - name: Install dependencies (Linux)

.github/workflows/test_without_exceptions_rtti.yml

@@ -23,7 +23,7 @@ jobs:
           - os: ubuntu-latest
             compiler: g++
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - name: Install dependencies (Linux)