The checkout v2, the version currently in use on CI, depends on node12
which is going to be removed soon. Let's use the latest version in order
to get rid of deprecation warnings in logs and remain compatible with
GitHun runners.

CI: Bump actions/checkout to v4

The macOS 13 runner image isn't shipped with pre-installed PostgreSQL
server. Even though it has Beta status, it'd be nice to support this
runner image too.

Reported-by: @baconcheese113
Fixes: #16

Add macOS 13 support

Unfortunately, the Windows runner has some PostgreSQL environment
variables set, which are neither set for Linux or macOS runners. This is
may be especially confusing since variables such as PGUSER or PGPASSWORD
may point to a user that doesn't exist.

Since one of the design decisions made previously is to restrain this
action from pointing to any user by default, we better unset these
environment variables to avoid confusion.

In addition to that change, let's also mention in the README file that
it's up to users to set connection parameters whatever way they prefer.

Reported-by: bkoelman
Fixes: #17

Unset PG* env vars except PGSERVICEFILE