use latest Ubuntu in CI workflows

[boegel]

fix for:

Running cvmfs_config setup
  sudo: cvmfs_config: command not found
  !!! github-action-cvmfs FAILED !!!
  cvmfs_config setup exited with 1

which occurs because apt repository can't be found anymore (since 21 June):

   Err:2 http://security.ubuntu.com/ubuntu focal-updates/main amd64 gdb amd64 9.2-0ubuntu1~20.04.1
    404  Not Found [IP: 40.81.13.82 80]
  Err:3 http://security.ubuntu.com/ubuntu focal-updates/main amd64 gdbserver amd64 9.2-0ubuntu1~20.04.1
    404  Not Found [IP: 40.81.13.82 80]
  E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/g/gdb/gdb_9.2-0ubuntu1~20.04.1_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
  E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/g/gdb/gdbserver_9.2-0ubuntu1~20.04.1_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
  E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

[eessi-bot-aws[bot]]

Instance eessi-bot-mc-aws is configured to build for:

• architectures: x86_64/generic, x86_64/intel/haswell, x86_64/intel/skylake_avx512, x86_64/amd/zen2, x86_64/amd/zen3, aarch64/generic, aarch64/neoverse_n1, aarch64/neoverse_v1
• repositories: eessi.io-2023.06-compat, eessi-hpc.org-2023.06-software, eessi-hpc.org-2023.06-compat, eessi.io-2023.06-software

[eessi-bot-aws[bot]]

Instance eessi-bot-mc-azure is configured to build for:

• architectures: x86_64/amd/zen4
• repositories: eessi.io-2023.06-software, eessi-hpc.org-2023.06-compat, eessi-hpc.org-2023.06-software, eessi.io-2023.06-compat

[eessi-build-deploy-bot-deucalion[bot]]

Instance boegel-bot-deucalion is configured to build for:

• architectures: aarch64/a64fx
• repositories: eessi.io-2023.06-software

[boegel]

@ocaisa ready to merge?