ci.yml: use actions/checkout@v4

This addresses the following warning from GitHub Actions:

    Node.js 16 actions are deprecated. Please update the following
    actions to use Node.js 20: actions/checkout@v3. For more information
    see:
    https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

A near-identical warning forced everyone to upgrade to v3 last year, so
this is some pointless churn, but let's just get it over with again...

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     env:
       CC: ${{ matrix.compiler }}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -29,7 +29,7 @@ jobs:
         compiler: [gcc, clang]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: uraimo/run-on-arch-action@v2.5.0
         with:
           arch: ${{ matrix.arch }}
@@ -51,7 +51,7 @@ jobs:
     env:
       CFLAGS: -Werror -DLIBDEFLATE_ENABLE_ASSERTIONS
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - run: cmake -B build -DLIBDEFLATE_BUILD_TESTS=1
     - run: cmake --build build --verbose
     - run: DESTDIR=build/install cmake --install build --verbose
@@ -73,7 +73,7 @@ jobs:
     env:
       CFLAGS: -Werror -DLIBDEFLATE_ENABLE_ASSERTIONS
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: msys2/setup-msys2@v2
       with:
         msystem: ${{matrix.sys}}
@@ -105,7 +105,7 @@ jobs:
         - {os: windows-2019, gen: "Visual Studio 16 2019", toolset: v142,    vs: Win32, vcpkg: x86-windows}
     runs-on: ${{matrix.os}}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: microsoft/setup-msbuild@v1.1
     - run: vcpkg install zlib:${{matrix.vcpkg}}
     - run: >
@@ -133,7 +133,7 @@ jobs:
           toolset: ClangCL
     runs-on: windows-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: microsoft/setup-msbuild@v1.1
     # Note: as per the CMake documentation, DESTDIR is unsupported on Windows.
     - run: >
@@ -147,7 +147,7 @@ jobs:
     name: Run clang static analyzer
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -159,7 +159,7 @@ jobs:
     name: Run shellcheck
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -173,7 +173,7 @@ jobs:
     env:
       CFLAGS: -Werror -DLIBDEFLATE_ENABLE_ASSERTIONS
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -203,7 +203,7 @@ jobs:
     env:
       CFLAGS: -Werror -DLIBDEFLATE_ENABLE_ASSERTIONS
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - run: |
         scripts/cmake-helper.sh \
             -DCMAKE_TOOLCHAIN_FILE="$ANDROID_NDK_LATEST_HOME"/build/cmake/android.toolchain.cmake \
@@ -217,7 +217,7 @@ jobs:
     name: Test building adler32.c and crc32.c with various flags
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update
@@ -268,7 +268,7 @@ jobs:
           sanitizer:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         sudo apt-get update