FIX: generates automatic slug for trashed channels (#19908)

Prior to this fix trashed channels would still prevent a channel with the same slug to be created. This commit generates a new slug on trash and frees the slug for future usage.

The format used for the slug is: `YYYYMMDD-HHMM-OLD_SLUG-deleted` truncated to the max length of a channel name.

plugins/chat/app/controllers/api/chat_channels_controller.rb

@@ -38,6 +38,13 @@ class Chat::Api::ChatChannelsController < Chat::Api
 
     begin
       ChatChannel.transaction do
+        channel_from_params.update!(
+          slug:
+            "#{Time.now.strftime("%Y%m%d-%H%M")}-#{channel_from_params.slug}-deleted".truncate(
+              SiteSetting.max_topic_title_length,
+              omission: "",
+            ),
+        )
         channel_from_params.trash!(current_user)
         StaffActionLogger.new(current_user).log_custom(
           "chat_channel_delete",