Drop support for EL7/RHEL7/SL7
Scientific Linux 7 (SL7, and the RHEL7 underlying it) is now severely deprecated at LIGO(see "LIGO Lab O4b Python2 and Scientific Linux 7 Security Policy for Computing Clusters"). We should drop support for SL7/EL7/RHEL7 at some point. (It's also the EL7 parts of CI that consistently take noticeably longer than anything else to finish.)