actions: update to latest provided versions

- checkout@v4: uses default runtime node20 rather than node16, avoiding
      a deprecation warning.
- macos-12: github actions deprecated macos-11 on 2024-06-28 [1]

[1] https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/#macos-11-deprecation-and-removal

.github/workflows/compile.yml

@@ -38,7 +38,7 @@ jobs:
       run: sudo apt-get install --no-install-recommends
            valgrind autoconf-archive
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
     - name: Run autoreconf
       run:
         autoreconf -i
@@ -84,10 +84,10 @@ jobs:
       run: test -z "$(git status --porcelain=v1)"
   macOS:
     name: macOS
-    runs-on: macOS-11
+    runs-on: macOS-12
     steps:
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
     - name: Install software
       run: brew install automake
     - name: Run autoreconf