BUG #18979: pg_upgrade to PG17 fails if max_slot_wal_keep_size is not set to -1

The following bug has been logged on the website:

Bug reference: 18979
Logged by: Jorge Solorzano
Email address: jorsol(at)gmail(dot)com
PostgreSQL version: 17.5
Operating system: Linux
Description:

Steps to Reproduce:
1. On a PostgreSQL 16 cluster, set max_slot_wal_keep_size = 500 (or any
non-default value).
2. Initdb a new PostgreSQL 17 cluster.
3. Copy the postgresql.conf from 16 to 17.
4. Attempt to perform a binary upgrade to PostgreSQL 17 using pg_upgrade
--check.
Expected Behavior:
pg_upgrade should automatically override max_slot_wal_keep_size to -1 as
required for upgrade mode.
Actual Behavior:
The upgrade fails with the following error:
command: "/usr/pgsql-17/bin/pg_ctl" -w -l
"/var/lib/pgsql/17/data/pg_upgrade_output.d/20250706T152559.441/log/pg_upgrade_server.log"
-D "/var/lib/pgsql/17/data" -o "-p 50432 -b -c synchronous_commit=off -c
fsync=off -c full_page_writes=off -c max_slot_wal_keep_size=-1 -c
listen_addresses='' -c unix_socket_permissions=0700 -c
unix_socket_directories='/var/lib/pgsql'" start >>
"/var/lib/pgsql/17/data/pg_upgrade_output.d/20250706T152559.441/log/pg_upgrade_server.log"
2>&1
waiting for server to start....2025-07-06 13:25:59.929 GMT [9439] LOG:
invalid value for parameter "max_slot_wal_keep_size": 500
2025-07-06 13:25:59.929 GMT [9439] DETAIL: "max_slot_wal_keep_size" must be
set to -1 during binary upgrade mode.
2025-07-06 15:25:59.930 CEST [9439] FATAL: configuration file
"/var/lib/pgsql/17/data/postgresql.conf" contains errors
stopped waiting
pg_ctl: could not start server
Additional Context:
While pg_upgrade does pass other required parameters like -c
max_slot_wal_keep_size=-1 on the command line when starting the new cluster
in upgrade mode, the value in postgresql.conf appears to override this,
leading to startup failure.
It would be helpful if pg_upgrade temporarily overrides the setting
correctly, regardless of the static config.