refactor!: remove HotBroadcaster

[sapphi-red]

Description

built on top of #19987

This PR removes HotBroadcaster type and changes the type of server.hot to NormalizedHotChannel.

• 17db814: simply reduces the code by using the fact that the number of channel is 1
• 9e7b2cf: correct the comment. server.hot is more like an alias to server.environments.client.hot instead of a separate concept
• ad9baa3: remove deprecated no-op HotBroadcaster.addChannel method
• ec6eb8e: deprecate HotBroadcaster.channels property. HotBroadcaster is deprecated but server.hot is not deprecated, maybe users won't expect this property to be removed.
• 53b8bc1: remove HotBroadcaster.channels property. If removing this property directly without a deprecation phase, this commit can be included. I guess it's fine to remove it directly. It's always [server.ws] and easy to replace. While I found server.hot used in many places, I only found hot.channels used in a single place.

I didn't deprecate server.hot as it's used in many places and keeping that won't be much burden.

[patak-dev]

I agree it is better to remove hot.channels in Vite 7
@AlemTuzlak, this would break react-router-devtools (here and in a few others places) as pointed out by @sapphi-red in the description. Maybe you could release a patch to make the access optional to avoid disruption when Vite 7 is released?

[AlemTuzlak]

@patak-dev Thank you so much for the heads up! On it!

[AlemTuzlak]

@patak-dev opened up a PR on react-router-devtools, mind checking the plugin file, I think this is all that is needed (I tested it out, all seems to work, but I'd like to confirm):
forge-42/react-router-devtools#208

[patak-dev]

Looks good to me Alem, thanks for the quick change!