Adds a config-optional endgame chat message (#72860)

## About The Pull Request

This basically does what we do for roundstart announcements, but for
round end.

With a delay between a round ending, the server rebooting, and a new
round starting, sometimes it feels like players would be more likely to
catch a roundstart when they know the previous game has just ended, and
not a few moments before the next one starts.

## Why It's Good For The Game

This idea was suggested to me several times by many people who don't
have good connections to servers and keep missing roundstart because
they just aren't given enough time to get on SS13.

I also included Round ID in this, so people who know what time they've
played a round can also now easily see which round it was, if they
wanted to go back to look at the logs for any reason they have.

## Changelog

🆑
config: There's now a config-optional announcer for a round ending.
/🆑

---------

Co-authored-by: Mothblocks <35135081+Mothblocks@users.noreply.github.com>

code/controllers/configuration/entries/general.dm

@@ -517,12 +517,21 @@
 	integer = FALSE
 
 /datum/config_entry/flag/irc_announce_new_game
-	deprecated_by = /datum/config_entry/string/chat_announce_new_game
+	deprecated_by = /datum/config_entry/string/channel_announce_new_game
 
 /datum/config_entry/flag/irc_announce_new_game/DeprecationUpdate(value)
 	return "" //default broadcast
 
 /datum/config_entry/string/chat_announce_new_game
+	deprecated_by = /datum/config_entry/string/channel_announce_new_game
+
+/datum/config_entry/flag/chat_announce_new_game/DeprecationUpdate(value)
+	return "" //default broadcast
+
+/datum/config_entry/string/channel_announce_new_game
+	default = null
+
+/datum/config_entry/string/channel_announce_end_game
 	default = null
 
 /datum/config_entry/string/chat_new_game_notifications