SHOW server_version \gset
SELECT CASE
           WHEN substring(current_setting('server_version'), '\d+')::int >= 17 THEN '17+'
           WHEN substring(current_setting('server_version'), '\d+')::int IN (15, 16) THEN '15_16'
           WHEN substring(current_setting('server_version'), '\d+')::int = 14 THEN '14'
           ELSE 'Unsupported version'
       END AS version_category;
 version_category
---------------------------------------------------------------------
 14
(1 row)

SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
\gset
\if :server_version_ge_15
\else
\q