Column active_time missing in postgres < 14

[MarcWort]

As @dbakit already said in #961 (comment) active_time is only available on postgres >=14.

Maybe there is a need for versioning pg versions in the collector package?

[sysadmind]

There is support for versions in the collector package. The instance struct has the postgres version for comparison.

https://github.com/prometheus-community/postgres_exporter/blob/master/collector/instance.go