adding clang 20 to our CI coverage. (#2739)

.github/workflows/ubuntu24-clang20.yml

@@ -0,0 +1,27 @@
+name: Ubuntu 24.04 CI (CLANG 20)
+
+on: [push, pull_request]
+
+jobs:
+  ubuntu-build:
+    if: >-
+      ! contains(toJSON(github.event.commits.*.message), '[skip ci]') &&
+      ! contains(toJSON(github.event.commits.*.message), '[skip github]')
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v4
+        with:
+          path: dependencies/.cache
+          key: ${{ hashFiles('dependencies/CMakeLists.txt') }}
+      - name: Install clang-20
+        run: |
+          sudo apt-get update -q -y
+          sudo apt-get install -y clang-20
+      - name: Use cmake
+        run: |
+          mkdir build &&
+          cd build &&
+          CXX=clang++-20 cmake -DSIMDJSON_DEVELOPER_MODE=ON ..  &&
+          cmake --build .   &&
+          ctest --output-on-failure -LE explicitonly -j