Rebase the version-specific SQL templates to target PostgreSQL 14 (#10054)

The base (default) SQL templates previously targeted PostgreSQL < 12.
Re-base them so the default target is 14 - the oldest supported server
version - by collapsing every version bucket <= 14 (11_plus, 12_plus,
13_plus, 14_plus and the old default) into a single `default`, keeping
per file the content a v14 server resolves today. Buckets for newer
versions (15_plus, 16_plus, 17_plus, 18_plus) are retained as overrides.

The transformation is behaviour-preserving for every server version >= 14:
template (and test-fixture) resolution is byte-identical before and after
for all supported versions, verified programmatically across every bucket
container and confirmed by the resql, ERD and Schema Diff suites against
PostgreSQL 18.

Also drop PostgreSQL/EDB Advanced Server 13 from the 9.16 supported-server
list and repoint the sqleditor explain_plan tests (which referenced the
removed 12_plus/13_plus buckets) at the new default template.

Closes #10050