[symbol-check] bump glibc to 2.27 and gcc to 8.3

Summary:
We don't really support Ubuntu < 18.04, and this will allow bumping libevent to 2.1.12 in D13453, which uses glibc 2.25 symbols.

Depends on D13459

Test Plan: gitian builds on CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13460