Adjust CI pipeline for apr21 release of FairSoft

Drop support for sl6
Change pipeline jobs in fairsoft2021_dev to use new base images
Temporarily change the paths to fairsoft/fairroot in all of the jobs to point to the new versions
Verify tests are running as expected
Right before merging fairsoft2021_dev into dev change symlinks in cvmfs to point to the new versions and revert pipeline jobs to using them
Right before merging fairsoft2021_dev into dev switch docker image tags in all of the jobs back to latest

Edited Oct 26, 2021 by Nikita Balashov

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