Upgrade GitHub Actions ubuntu version (22 -> 24)

[lukehesluke]

In this PR: #696, I fixed ubuntu version from ubuntu-latest to ubuntu-22.04. This is because a change appears to have happened in Ubuntu which leads to this error:

Process terminated. Couldn't find a valid ICU package installed on the system. Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.

(link to the whole run)

Downgrading to 22.04 fixed this

What's going on

This is what I have gathered so far:

• Adding this step to the tests job (in .github/workflows/reference-implementation.yml) before the Install OpenActive.Server.NET dependencies step may fix it as it will install an ICU package:

  # These are required for .NET globalization
  - name: Install ICU packages
    run: |
      sudo apt-get update
      sudo apt-get install -y libicu-dev

• Or alternatively setting:

  env:
    DOTNET_SYSTEM_GLOBALIZATION_INVARIANT: 1

  in the test may fix it but will turn off globalization (https://learn.microsoft.com/en-us/dotnet/core/runtime-config/globalization)

But it might also be worth finding out why a Ubuntu upgrade might have caused this. Is everyone having to do one of these additional steps in order to run .NET now? Maybe there's a more canonical solution