Disable room list publication by default (#18175)

This is in line with our general policy of ensuring that the default
config is reasonably locked down.

SyTest PR to fix tests: https://github.com/matrix-org/sytest/pull/1396

docs/usage/configuration/config_documentation.md

@@ -4245,8 +4245,8 @@ unwanted entries from being published in the public room list.
 
 The format of this option is the same as that for
 [`alias_creation_rules`](#alias_creation_rules): an optional list of 0 or more
-rules. By default, no list is provided, meaning that all rooms may be
-published to the room list.
+rules. By default, no list is provided, meaning that no one may publish to the
+room list (except server admins).
 
 Otherwise, requests to publish a room are matched against each rule in order.
 The first rule that matches decides if the request is allowed or denied. If no
@@ -4272,6 +4272,10 @@ Note that the patterns match against fully qualified IDs, e.g. against
 of `alice`, `room` and `abcedgghijk`.
 
 
+_Changed in Synapse 1.126.0: The default was changed to deny publishing to the
+room list by default_
+
+
 Example configuration:
 
 ```yaml