[Update] Better instructions joint PG/Timescale upgrades

[mfreed]

Describe the update We should have better instructions how to upgrade PG and TimescaleDB concurrently, particularly when there are version interactions, e.g., PG12 isn't supported prior to 1.7, so if coming from PG10/TimescaleDB 1.6, there is a careful order of things that must be done (first upgrade Timescale to 1.7, then pg_upgrade PG to 12). This is also true with TimescaleDB 2.0 drops support for PG 9.6 and 10.

Location(s) https://docs.timescale.com/latest/using-timescaledb/update-db

How soon is this needed? Should be done soon to reflect 1.7.

[mfreed]

See, e.g., github issue https://github.com/timescale/timescaledb/issues/1836