[3.x] Update PostgreSQLSchemaManager to set correct config key value

[nthndnn]

In Laravel 9.0, the Postgres schema config key has been replaced by search_path (https://laravel.com/docs/9.x/upgrade#postgres-schema-configuration).

This PR updates the PostgreSQLSchemaManager to check which config value is in use and sets it accordingly. It might be worth setting both no matter whether the key exists or not, but I thought this would be the best solution.

[nthndnn]

I've updated this to use version_compare as this has been done elsewhere and looks like a more readable solution 😄

[stancl]

The test could probably use version_compare() as well

[nthndnn]

@stancl cool, wasn't sure whether app()->version() was available in the tests

[stancl]

Looks great now, thanks a lot for the PR!