robotology-superbuild master branch stop support for Ubuntu 18.04 with apt-provided dependencies

[traversaro]

Hi @robotology/everyone!

Due to the lack of support in downstream projects, the robotology-superbuild master branch will stop supporting (i.e. check compilation via Continuous Integration) Ubuntu 18.04 with apt-provided dependencies.

This will mean that 2021.11.* will be the last release with Ubuntu 18.04 support, and 2022.02.0 will be the first release with no Ubuntu 18.04 support.

Existing Ubuntu 18.04 users can continue to use the 2021.11.0 release with apt-provided dependencies, or switch to use conda to install robotology-superbuild's dependencies or to install directly the binaries of robotology-superbuild projects.

If you are curious regarding this decision, see the discussion in robotology/robotology-superbuild#481 .