BUG #18988: DROP SUBSCRIPTION locks not-yet-accessed database

The following bug has been logged on the website:

Bug reference:      18988
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 18beta1
Operating system:   Ubuntu 24.04
Description:

The following script:
createdb ndb
echo "
CREATE SUBSCRIPTION testsub CONNECTION 'dbname=ndb' PUBLICATION testpub WITH
(connect = false);
" | psql

echo "
DROP SUBSCRIPTION testsub
" | psql &
sleep 1
timeout 30 psql ndb -c "SELECT 1" || echo "TIMEOUT"

makes DROP SUBSCRIPTION stuck on waiting for a connection to drop a slot,
while this connection is waiting for a lock for relation 6100
(pg_subscription), locked by DROP SUBSCRIPTION:
law      1545967 1545946  0 21:10 ?        00:00:00 postgres: law regression
[local] DROP SUBSCRIPTION
law      1545968 1545946  0 21:10 ?        00:00:00 postgres: walsender law
ndb [local] startup waiting

With debug_discard_caches = 1 or under some lucky circumstances (I
encountered these), this leads to inability to connect to any database.

Reproduced on REL_13_STABLE .. master.