GEC-219 Drop Ubuntu 20 support; Support Boost 1.89

Maik Nijhuisrequested to merge
gec-219-support-boost-1.89 into master
  • Remove Ubuntu 20.04 docker images and CI job.
  • Remove date_time and system from the Boost COMPONENTS list, since they're header-only now.
  • Raise the minimum Boost version to 1.73.
Edited by Maik Nijhuis

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