Add `humble` to Linux CI

[hoffmann-stefan]

depends on https://github.com/ros2-dotnet/dotnet_cmake_module/pull/4 👈 already merged

This started as do a minimal change to test Ubuntu 22.04, but did involve some more tasks on the way:

• Add matrix with ubuntu-20.04 and ubuntu-22.04.
• Foxy was not supported on ubuntu-20.04 👉 Add humble to ci.
• .NET Core 2.1 is not supported on ubuntu-22.04, use .NET 6 instead.
  • But tests did run on netcoreapp2.0 by default 👉 Make PR in dotnet_cmake_module to be able to specify the TARGET_FRAMEWORK for tests.
  • https://github.com/ros2-dotnet/dotnet_cmake_module/pull/4
  • [x] This needs to be merged first before this can be merged.
• As turned out to be more about the different ros distributions as about the Ubuntu versions. 👉 Change the matrix to be based on the ros distribution.
  • Took some inspiration from ros2_rust.
  • https://github.com/ros2-rust/ros2_rust/blob/main/.github/workflows/rust.yml
• Include the fix for ubuntu-22.04 with libdl.so[.2].
  • see https://github.com/ros2-dotnet/ros2_dotnet/issues/97
  • Reproducing the bug in CI started this all.
• Change the Linux foxy build to use .NET 6 as .NET Core 2.1 is not supported anymore.
• While were at it add rolling as a bonus
  • Will be a follow-up PR as it doesn't build right now, needs furhter investigation.

Overview of supported .NET versions on Ubuntu

https://learn.microsoft.com/en-us/dotnet/core/install/linux-ubuntu#supported-distributions

Going with .NET 6 for Ubuntu 22.04 from the Microsoft package feed. 👉 Causes issues with the already installed .NET 6 SDK in the github actions ubuntu-22.04 image. So use the already installed one.