Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6
         with:
@@ -48,7 +48,7 @@ jobs:
   coverage:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Cache LLVM and Clang
         id: cache-llvm
         uses: actions/cache@v4
@@ -88,7 +88,7 @@ jobs:
       matrix:
         check: ["pylint", "pycodestyle", "black", "mypy"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Cache LLVM and Clang
         id: cache-llvm
         uses: actions/cache@v4
@@ -118,7 +118,7 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Cache LLVM and Clang
         id: cache-llvm
         uses: actions/cache@v4
@@ -150,7 +150,7 @@ jobs:
       matrix:
         sphinx-version: ["four"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Cache LLVM and Clang
         id: cache-llvm
         uses: actions/cache@v4

.github/workflows/release.yml

@@ -16,7 +16,7 @@ jobs:
     permissions:
       id-token: write
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Python
         uses: actions/setup-python@v6
         with: