Fix serial speed DD configuration & migrate keyboards (#25546)

* Fix serial speed DD configuration

- Fixes incorrect SOFT_SERIAL_SPEED mapping
- Renames key split.soft_serial_speed -> split.serial.speed
- Migrates keyoards that configure this, and remove configuration from keyboards that do not differ from the default behaviour
- Add deprecation notice and migration support

keyboards/unikeyboard/diverge3/config.h

@@ -17,10 +17,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #pragma once
 
-#ifndef SELECT_SOFT_SERIAL_SPEED
-#define SELECT_SOFT_SERIAL_SPEED 3
-#endif
-
 /* If defined, GRAVE_ESC will always act as ESC when CTRL is held.
  * This is useful for the Windows task manager shortcut (ctrl+shift+esc).
  */

keyboards/unikeyboard/diverge3/keyboard.json

@@ -34,7 +34,8 @@
   "split": {
     "enabled": true,
     "serial": {
-      "pin": "D0"
+      "pin": "D0",
+      "speed": 3
     }
   },
   "development_board": "promicro",