Fix demo script on ipv6-supported boxes (#6229)

The synapse demo was a bit flakey in terms of supporting federation. It turns out that if your computer resolved `localhost` to `::1` instead of `127.0.0.1`, the built-in federation blacklist specified in `start.sh` would still block it, since it contained an entry for `::/127`. Removing this no longer prevents Synapse from contacting `::1`, federation works again on these boxes.

changelog.d/6229.bugfix

@@ -0,0 +1 @@
+Prevent the demo Synapse's from blacklisting `::1`.

demo/start.sh

@@ -77,14 +77,13 @@ for port in 8080 8081 8082; do
 
         # Reduce the blacklist
         blacklist=$(cat <<-BLACK
-		# Set the blacklist so that it doesn't include 127.0.0.1
+		# Set the blacklist so that it doesn't include 127.0.0.1, ::1
 		federation_ip_range_blacklist:
 		  - '10.0.0.0/8'
 		  - '172.16.0.0/12'
 		  - '192.168.0.0/16'
 		  - '100.64.0.0/10'
 		  - '169.254.0.0/16'
-		  - '::1/128'
 		  - 'fe80::/64'
 		  - 'fc00::/7'
 		BLACK