Migrate remaining split.soft_serial_pin to split.serial.pin (#25046)

* Migrate keyboards/bastardkb

* Migrate keyboards/handwired

* Migrate keyboards/helix

* Fix duplicate serial key

keyboards/helix/pico/info.json

@@ -10,7 +10,9 @@
     },
     "split": {
         "enabled": true,
-        "soft_serial_pin": "D2"
+        "serial": {
+            "pin": "D2"
+        }
     },
     "tapping": {
         "term": 100

keyboards/helix/rev2/info.json

@@ -15,7 +15,9 @@
     "diode_direction": "COL2ROW",
     "split": {
         "enabled": true,
-        "soft_serial_pin": "D2",
+        "serial": {
+            "pin": "D2"
+        },
         "transport": {
             "sync": {
                 "indicators": true,

keyboards/helix/rev3_4rows/keyboard.json

@@ -33,7 +33,9 @@
   },
   "split": {
     "enabled": true,
-    "soft_serial_pin": "D2"
+    "serial": {
+        "pin": "D2"
+     }
   },
   "ws2812": {
     "pin": "D3"

keyboards/helix/rev3_5rows/keyboard.json

@@ -99,7 +99,9 @@
   },
   "split": {
     "enabled": true,
-    "soft_serial_pin": "D2"
+    "serial": {
+        "pin": "D2"
+    }
   },
   "ws2812": {
     "pin": "D3"