GitHub-CI: Bump actions/checkout from 5 to 6.

* .github/workflows/codeql-analysis.yaml, .github/workflows/make-alpine.yaml,
.github/workflows/make-cygwin.yaml, .github/workflows/make-macos.yaml,
.github/workflows/make-mingw.yaml, .github/workflows/make-ubuntu.yaml: Bump
actions/checkout from 5 to 6.

See: #36

Author: dependabot[bot]

.github/workflows/codeql-analysis.yaml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install dependencies
         run: |
@@ -151,7 +151,7 @@ jobs:
           msystem: ${{ matrix.msystem }}
 
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: bootstrap
         run: GNULIB_URL=https://github.com/coreutils/gnulib.git ./bootstrap

.github/workflows/make-alpine.yaml

@@ -26,7 +26,7 @@ jobs:
           cat /proc/cpuinfo
 
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install dependencies
         uses: jirutka/setup-alpine@v1

.github/workflows/make-cygwin.yaml

@@ -39,7 +39,7 @@ jobs:
       - name: checkout repository
         # This must be done before installing Cygwin because their git is
         # incompatible with this action.
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install Cygwin build environment
         uses: cygwin/cygwin-install-action@v6

.github/workflows/make-macos.yaml

@@ -44,7 +44,7 @@ jobs:
 
     steps:
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install dependencies
         # This step sometimes takes long and still recovers. But sometimes it

.github/workflows/make-mingw.yaml

@@ -116,7 +116,7 @@ jobs:
           msystem: ${{ matrix.msystem }}
 
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: prepare ccache
         # create key with human readable timestamp

.github/workflows/make-ubuntu.yaml

@@ -55,7 +55,7 @@ jobs:
         run: lscpu
 
       - name: checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: install dependencies
         env: