postgresql-common: do not call postinst_check_catversion() if create_main_cluster = no

Hi,

In /usr/share/postgresql-common/maintscripts-functions,
configure_version() calls postinst_check_catversion() regardless of
whether createcluster.conf says create_main_cluster = no

I saw this after 1) initially installing postgres; then later 2) setting
create_main_cluster=no and then (re)running initdb myself; and then, 3)
updating to a daily snapshot (16.1-0.pgdg120+~20231019.0500.gcfa4eba).

BTW, thanks for providing daily snapshots.

-- 
Justin