postgres: only force a generic query plan where EXPLAIN is available (#4286)

The query macros set `plan_cache_mode = 'force_generic_plan'` on their
describe connection (#3541) to keep nullability inference from being skewed
by the NULL placeholder arguments. It was issued inside a `DO` block, which
CockroachDB rejects at parse time -- `SET` inside a function body, SQLSTATE
0A000 -- breaking the query macros against CockroachDB since 0.9.0 (#4274).

The setting only benefits the EXPLAIN-based inference, which already runs
only when `is_explain_available()` is true (false for CockroachDB,
Materialize and QuestDB). Gate the setting on that same check so it is never
issued on a database that cannot use it, replacing the `pg_settings` probe
with a `server_version_num() >= 12` guard.