Support PostgreSQL 15 and higher

[japinli]

Ths pg_is_in_backup() and pg_backup_start_time() have been removed in PostgreSQL 15. See: https://www.postgresql.org/docs/release/15.0/

Remove {pg_primaryonly,pg_replicaonly}{is_in_backup,backup_time} metrics for PostgreSQL 15 and higher.

[Vonng]

makes sense, I will release a v0.6.0 today

[Vonng]

Thanks a lot, already did that: https://github.com/Vonng/pg_exporter/commit/85ec39b966e9ef262f8fa35f3b515896534c3204