BUG #19075: pg_upgradecluster does not copy conf.d/* during cluster upgrade

The following bug has been logged on the website:

Bug reference: 19075
Logged by: Sergey Belyashov
Email address: sergey(dot)belyashov(at)gmail(dot)com
PostgreSQL version: 17.6
Operating system: Debian bookworm x86_64
Description:

I try to upgrade DB postgresql 17 to 18. So I install postgresql-18 (from
PostgreSQL repo) and then run "pg_upgradecluster -m link 17 main"...
It fails on checking wal_level of the new cluster - wal_level should be
logical but it is replica.
Reason: I have redefined default settings in the extra config file places in
the /etc/postgresql/17/main/conf.d/local.conf.
So I have wal_level=logical defined there, but not in the
/etc/postgresql/17/main/postgresql.conf.
If I redefine it in the /etc/postgresql/17/main/postgresql.conf then
pg_upgradecluster succeed.