Set minimum required glibc to 2.31.
The glibc 2.31 branch is still maintained:
https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/release/2.31/master.

Remove the stack-protector check from test-security-check, as the test
no-longer fails, and given the control we have of the end, the actual
security-check test seems sufficient (this might also be applied to some
of the other checks).

Drops runtime support for Ubuntu Bionic 18.04 and RHEL-8 from the release binaries.