Fix SQL error when Postgres' datestyle is not MDY
See https://gitlab.com/gitlab-org/gitlab-exporter/-/issues/94: use TO_TIMESTAMP(0) rather than '01-01-1970' for epoch (don't assume datestyle is MDY).
See https://gitlab.com/gitlab-org/gitlab-exporter/-/issues/94: use TO_TIMESTAMP(0) rather than '01-01-1970' for epoch (don't assume datestyle is MDY).