Update gitlab-ci-*.yml files
Generally bring the gitlab-ci files up to date with recent distributions.
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add clang-11; delete clang-6, delete clang-7 -
add gcc-10; delete gcc-4.9 -
add Fedora 32, 33; delete Fedora 27-30 -
add Ubuntu 20.04; fix Ubuntu-rolling libgsl23 dependency
Edited by Tommaso Pecorella