Allow pg_query v6

What does this MR do?

pg_query v6 updates libpg_query tag and checksum:

Diff: https://my.diffend.io/gems/pg_query/5.1.0/6.0.0

This is required in order to bump the gem on https://gitlab.com/gitlab-org/gitlab

Could not find compatible versions

Because gitlab-labkit >= 0.35.0 depends on pg_query >= 4.2.3, < 6.0
  and Gemfile depends on pg_query ~> 6.0.0,
  gitlab-labkit >= 0.35.0 cannot be used.
So, because Gemfile depends on gitlab-labkit ~> 0.36.0,
  version solving has failed.