Fix our PG_VERSION_STRING_MAX_LENGTH to host more. (#353)

The current debian Postgres version string is longer than we anticipated:

    "16beta1 (Debian 16~beta1-2.pgdg+~20230605.2256.g3f1aaaa)"

In passing, also make sure that `pgcopydb ping` sets the right GUCs even
when using Postgres 9.5, and allow more GUCs for that version too.