ci: Test cmake_minimum_required

Ensures project configures with 3.13 since it's very easy to
write code that doesn't respect the minimum.

.github/workflows/cmake.yml

@@ -0,0 +1,29 @@
+# Note: CMake support is community-based. The maintainers do not use CMake
+# internally.
+
+name: cmake
+
+on:
+    push:
+    pull_request:
+        branches:
+            - main
+
+permissions:
+    contents: read
+
+jobs:
+  cmake_min:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        config: ["Debug", "Release"]
+    steps:
+      - uses: actions/checkout@v3
+      - uses: lukka/get-cmake@latest
+        with:
+          cmakeVersion: 3.13.0
+      - name: Configure
+        run: cmake -S . -B build -DCMAKE_BUILD_TYPE=${{ matrix.config }} -G "Ninja"
+      - name: Build
+        run: cmake --build build --config ${{ matrix.config }}