Maximum supported PostgreSQL version

What does this MR do?

[1] Some code quality fixes while addressing [2]

[2] Customers regularly ask whether they can run a major release of PostgreSQL that is higher than our documented minimum

For example (internal slack links):

• 2023-10-23
• 2023-10-18
• 2023-11-04

Adding a maximum version to the docs helps to steer folks away from running much, much later versions and provides some clearer steer and logic to address this question when this arises.

Since we only specify a minimum, customers read this as "there is no maximum". So, while specifying a maximum may appear to create a support headache, Aspirin was already on prescription.

Related issues

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