I am also getting this error about /usr/bin/pihole-FTL: /lib/x86_64-linux-gnu/libc.so.6: version GLIBC_2.28 not found (required by /usr/bin/pihole-FTL) after updating on Ubuntu 18.04. It causes the pihole-FTL service to fail and no DNS resolution.
I have the same problem pihole-FTL[7186]: /usr/bin/pihole-FTL: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /usr/bin/pihole-FTL)
after upgrade pihole.