CI: Force CMake minimum version for mbedtls.

mbedtls specifies a minimum version of 2.8.12. This causes an error:

    Compatibility with CMake < 3.5 has been removed from CMake.

Use -DDCMAKE_POLICY_VERSION_MINIMUM as suggested.

Author: Gadgetoid

.github/workflows/cmake.yml

@@ -19,7 +19,7 @@ jobs:
           - os: ubuntu-24.04
             name: Linux
             cache-key: linux
-            cmake-args: '-DPIMORONI_PICO_PATH=$GITHUB_WORKSPACE/pimoroni-pico -DPICO_SDK_PATH=$GITHUB_WORKSPACE/pico-sdk -DCMAKE_INSTALL_PREFIX=$GITHUB_WORKSPACE/install'
+            cmake-args: '-DPIMORONI_PICO_PATH=$GITHUB_WORKSPACE/pimoroni-pico -DPICO_SDK_PATH=$GITHUB_WORKSPACE/pico-sdk -DCMAKE_INSTALL_PREFIX=$GITHUB_WORKSPACE/install -DCMAKE_POLICY_VERSION_MINIMUM=3.13...3.27'
             apt-packages: clang-tidy gcc-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib
 
     runs-on: ${{matrix.os}}