Managing PG configuration across major Postgres versions.

There are various Postgres configuration options that are added/removed/renamed across major Postgres versions. We need to come up with a sane strategy for organizing and maintaining configuration to accommodate past, present and future generations of Postgres.

Quick example:

In Postgres 12, we have `wal_keep_segments` which is used to specify the minimum number of past log file segments kept in the pg_wal directory.

In Postgres 13, this has been "renamed" to `wal_keep_size`.

> This determines how much WAL to retain for standby servers. It is specified in megabytes, rather than number of files as with the old parameter. If you previously used wal_keep_segments, the following formula will give you an approximately equivalent setting:
`wal_keep_size = wal_keep_segments * wal_segment_size (typically 16MB)
`