Migrate to the ubuntu-24.04 GitHub Actions runner

The ubuntu-20.04 runner is due to be deprecated on 2025-02-01 and
unsupported by 2025-04-01.

The ubuntu-24.04 runner doesn't include cbindgen so install it using
apt.

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   rustfmt:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
 
@@ -21,7 +21,7 @@ jobs:
       matrix:
         os:
           - windows-2019
-          - ubuntu-20.04
+          - ubuntu-24.04
 
     runs-on: ${{ matrix.os }}
 
@@ -38,6 +38,9 @@ jobs:
             target/
           key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
 
+      - name: Install cbindgen
+        run: cargo install cbindgen --version 0.27.0
+
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
         if: runner.os == 'Linux'