How to send a message on the bot startup to every server it is in?

First, to answer your question about default_channel: Since about June 2017, Discord no longer defines a "default" channel, and as such, the default_channel element of a server is usually set to None.

Next, by saying discord.Server.default_channel, you're asking for an element of the class definition, not an actual channel. To get an actual channel, you need an instance of a server.

Now, to answer the original question, which is to send a message to every channel, you need to find a channel in the server that you can actually post messages in:

python
    @bot.event
    async def on_ready():
        for server in bot.servers: 
            # Spin through every server
            for channel in server.channels: 
                # Channels on the server
                if channel.permissions_for(server.me).send_messages:
                    await bot.send_message(channel, "...")
                    # So that we don't send to every channel:
                    break