Migrate split.soft_serial_pin to split.serial.pin 0-H (#24155)

keyboards/boardsource/lulu/avr/keyboard.json

@@ -8,7 +8,9 @@
     "rows": ["C6", "D7", "E6", "B4", "B5"]
   },
   "split": {
-    "soft_serial_pin": "D2"
+    "serial": {
+      "pin": "D2"
+    }
   },
   "ws2812": {
      "pin": "D3"

keyboards/boardsource/microdox/v1/keyboard.json

@@ -10,7 +10,9 @@
     "device_version": "1.0.0"
   },
   "split": {
-    "soft_serial_pin": "D2"
+    "serial": {
+      "pin": "D2"
+    }
   },
   "rgblight": {
     "sleep": true,

keyboards/boardsource/microdox/v2/keyboard.json

@@ -19,7 +19,6 @@
     ]
   },
   "split": {
-    "soft_serial_pin": "D3",
     "matrix_pins": {
       "right": {
         "cols": ["F4", "B1", "D7", "C6", "B3"],
@@ -32,6 +31,9 @@
           {"pin_a": "B6", "pin_b": "B2"}
         ]
       }
+    },
+    "serial": {
+      "pin": "D3"
     }
   },
   "rgb_matrix": {

keyboards/boardsource/unicorne/keyboard.json

@@ -20,9 +20,9 @@
     },
     "split": {
         "enabled": true,
-        "soft_serial_pin": "GP0",
         "serial": {
-            "driver": "vendor"
+            "driver": "vendor",
+            "pin": "GP0"
         },
         "transport": {
             "sync": {