Fix missing development_board schema entry (#18050)

data/schemas/keyboard.jsonschema

@@ -34,7 +34,7 @@
         },
         "development_board": {
             "type": "string",
-            "enum": ["promicro", "elite_c", "proton_c", "kb2040", "promicro_rp2040", "blok", "bit_c_pro", "bluepill", "blackpill_f401", "blackpill_f411"]
+            "enum": ["promicro", "elite_c", "proton_c", "kb2040", "promicro_rp2040", "blok", "bit_c_pro", "stemcell", "bluepill", "blackpill_f401", "blackpill_f411"]
         },
         "pin_compatible": {
             "type": "string",