GH-45478: [CI][C++] Drop support for Ubuntu 20.04 · apache/arrow@cb38270

Canceling since a higher priority waiting request for 'apache/arrow-ci-cpp-ubuntu-20.04-R' exists

The operation was canceled.

Cache not found for keys: r-40-ccache-mingw-ucrt64-7687513f4898927077960306e885edd3c7c0114fb2fe0d98386abbcdd9095029-13299353891, r-40-ccache-mingw-ucrt64-7687513f4898927077960306e885edd3c7c0114fb2fe0d98386abbcdd9095029-, r-40-ccache-mingw-ucrt64-