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.
Without ffile-prefix-map, the debug symbols will contain paths for the
guix store which will include the hashes of each package. However, the
hash for the same package will differ when on different architectures.
In order to be reproducible regardless of the architecture used to build
the package, map all guix store prefixes to something fixed, e.g. /usr.
We might be able to drop this in favour of using --with-nonshared-cflags
when we being using newer versions of glibc.
