Raise default PostgreSQL shared buffers minimum to 256 MB

What does this MR do?

Previously the Docker container set the PostgreSQL shared buffers to a minimum of 1 MB to avoid using the default of 25% of the node's memory. While that meager value worked previously, this value is not only overly conservative but it is now insufficient for background migrations that have a large number of partitioned tables.

From #8682 (comment 2058612975) we see that the 2 years of audit_events needs about 20 MB just to query the table metadata. The 1 MB value failed with no unpinned buffers available because PostgreSQL needs more shared buffers for query planning.

Raise the default to 256 MB to provide enough headroom for future tables and to ensure PostgreSQL effectively caches data. Update the documentation in the template accordingly.

Related issues

Relates to #8682 (closed)

Checklist

See Definition of done.

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