.github: switch from actions/checkout@v3 to actions/checkout@v4

This fixes the following diagnostic warning:

  Node.js 16 actions are deprecated.
  Please update the following actions to use Node.js 20: actions/checkout@v3

Link: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
   whitespace-errors:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: check
       run: git diff-index --check --cached 4b825dc642cb6eb9a060e54bf8d69288fbee4904
 
@@ -16,7 +16,7 @@ jobs:
       CC: gcc-13
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -30,7 +30,7 @@ jobs:
       CC: gcc-12
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -44,7 +44,7 @@ jobs:
       CC: gcc-11
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -58,7 +58,7 @@ jobs:
       CC: gcc-10
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -72,7 +72,7 @@ jobs:
       CC: gcc
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -86,7 +86,7 @@ jobs:
       CC: gcc-8
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -100,7 +100,7 @@ jobs:
       CC: clang-15
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -114,7 +114,7 @@ jobs:
       CC: clang-14
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -128,7 +128,7 @@ jobs:
       CC: clang-13
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -142,7 +142,7 @@ jobs:
       CC: clang-12
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -156,7 +156,7 @@ jobs:
       CC: clang-11
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -170,7 +170,7 @@ jobs:
       CC: clang
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -184,7 +184,7 @@ jobs:
       CC: clang-9
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies
@@ -198,7 +198,7 @@ jobs:
       CC: clang-8
       TARGET: x86_64
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
     - name: install dependencies