Qt 5.15 LTS offline installers broken for RHEL 7

Beginning with 5.15.17, the Qt 5.15 LTS offline installers available for download from the Qt Customer Portal do not work for RHEL 7, which is a supported platform.

RHEL 7.9 supplies glibc 2.17. The offline installers for Qt 5.15.17 and Qt 5.15.18 appear to have been built against a more recent version. Qt 5.15.16 and earlier seem to be unaffected. Note that the C++ libraries are also an issue.