Prohibit max_slot_wal_keep_size to value other than -1 during upgrade.
We don't want existing slots in the old cluster to get invalidated during
the upgrade. During an upgrade, we set this variable to -1 via the command
line in an attempt to prevent such invalidations, but users have ways to
override it. This patch ensures the value is not overridden by the user.
Author: Kyotaro Horiguchi
Reviewed-by: Peter Smith, Alvaro Herrera
Discussion: http://postgr.es/m/
20231027.115759.
2206827438943188717.horikyota.ntt@gmail.com