guix: use glibc 2.31

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.

contrib/guix/patches/glibc-guix-prefix.patch

@@ -0,0 +1,16 @@
+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.
+
+--- a/Makeconfig
++++ b/Makeconfig
+@@ -1007,6 +1007,7 @@ object-suffixes :=
+ CPPFLAGS-.o = $(pic-default)
+ # libc.a must be compiled with -fPIE/-fpie for static PIE.
+ CFLAGS-.o = $(filter %frame-pointer,$(+cflags)) $(pie-default)
++CFLAGS-.o += `find /gnu/store -maxdepth 1 -mindepth 1 -type d -exec echo -n " -ffile-prefix-map={}=/usr" \;`
+ libtype.o := lib%.a
+ object-suffixes += .o
+ ifeq (yes,$(build-shared))