Update CI Pipeline configuration

This MR makes the following changes to the CI pipeline:

• uses the needs keyword for concurrent processing of jobs (see https://docs.gitlab.com/ee/ci/pipelines/pipeline_architectures.html#directed-acyclic-graph-pipelines).
• provide a unit test report to GitLab, this makes identifying failed tests easier. (🤣 needs ctest 3.21, https://cmake.org/cmake/help/v3.21/release/3.21.html#ctest)
• remove ubuntu16 (EOL), add ubuntu20