Move Linux arm64 CI from dead Cirrus CI to ubuntu-24.04-arm

Cirrus CI stopped reporting check runs on develop sometime after
d10879bca (2026-05-26); every commit since has only github-actions
check runs, so .cirrus.yml silently lost its only consumer while
README.md, FILES.md and the QA page kept advertising the coverage.

Add a ci_test_arm64 job to ubuntu.yml using the same pinned
actions/checkout and lukka/get-cmake actions as the other jobs, on the
native ubuntu-24.04-arm runner, with a step that confirms uname -m
reports aarch64. Delete .cirrus.yml and its README badge and FILES.md
section, and update the QA page's arm64 row.

Part of #5715

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -403,3 +403,21 @@ jobs:
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
         run: cmake --build build --target ${{ matrix.target }}
+
+  # Native Linux ARM64 runner with the latest GCC. The ubuntu-24.04-arm label is
+  # only available for public repositories.
+  ci_test_arm64:
+    runs-on: ubuntu-24.04-arm
+    container: gcc:latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@fffaaafeea488556c2c12dad60690008bc1caacb # v4.4.2
+      - name: Check that the job runs natively on arm64
+        run: uname -m && test "$(uname -m)" = aarch64
+      - name: Run CMake
+        run: cmake -S . -B build -DJSON_CI=On
+      - name: Build
+        run: cmake --build build --target ci_test_compiler_default