Use port 80 in placeholder bridge addresses

I did not test this change with a full browser build. I only tested it by changing the prefs extensions.torlauncher.default_bridge.meek-azure.1 and extensions.torlauncher.default_bridge.snowflake.1, and toggling the "This computer goes through a firewall that only allows connections to certain ports" setting. Without this change, meek-azure and Snowflake don't work; with this change, they do.

Closes #40624 (closed).