README: postgresql-client-12 no longer contained

Author: kkimurak

README.md

@@ -259,7 +259,6 @@ Configuring gitlab::database
 
 Please note furthermore, that only combatible versions of the `postgresql-client` to GitLab are shipped with this image. Currently these belogn to
 
-- `postgresql-client-12`,
 - `postgresql-client-13`,
 - `postgresql-client-14`,
 - and `postgresql-client-15`.