Upgrade PgBouncer to v1.12.0

This release fixes the SHOW METRICS problem described in https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7726.

Full details of changes: https://github.com/pgbouncer/pgbouncer/releases/

One change that we may need to double check works across all platforms:

Remove support for Libevent version 1.x. Libevent 2.x is now required. Libevent is now detected using pkg-config.