CI: Bump ubuntu runner to 24.04.

Deprecation began today and ubuntu-20.04 will be unavailable from 2025-04-15.

Don't use "ubuntu-latest" since CI might quietly break in future.

Author: Gadgetoid

.github/workflows/cmake.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-20.04
+          - 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'