Drop support for postgres versions older than 10

[rnaveiras]

According to https://www.postgresql.org/support/versioning/, 9.6 reached end-of-live on November 11, 2021

• Drop support for versions older than 10
• Review queries across all the collectors

[rnaveiras]

See #115

[ahjmorton]

From looking at the usage of Version as part of #115 it seems the only collector that interrogates the version is the one I'm changing for pg_stat_replication so will drop PG 9 support from that.

Assuming the other collectors are already working on PG 10+ that suggests the other queries at least work on PG 10+.

Reckon we can close this out if we merge #115 ?