[Build] Change to SDK 20190703 to improve WiFi

See:
- esp8266/Arduino#6724 (comment)
- esp8266/Arduino#5784 (comment)

Author: TD-er

platformio_core_defs.ini

@@ -98,12 +98,12 @@ lib_ignore                = ESP32_ping, ESP32WebServer, IRremoteESP8266, Heatpum
 [core_2_6_0]
 platform                  = https://github.com/Jason2866/platform-espressif8266.git#core_2_6_0
 build_flags               = ${esp82xx_2_6_x.build_flags} 
-                            -DPIO_FRAMEWORK_ARDUINO_ESPRESSIF_SDK22x_191105
+                            -DPIO_FRAMEWORK_ARDUINO_ESPRESSIF_SDK22x_190703
 
 [core_2_6_1]
 platform                  = https://github.com/Jason2866/platform-espressif8266.git#core_2_6_1
 build_flags               = ${esp82xx_2_6_x.build_flags} 
-                            -DPIO_FRAMEWORK_ARDUINO_ESPRESSIF_SDK22x_191105
+                            -DPIO_FRAMEWORK_ARDUINO_ESPRESSIF_SDK22x_190703
 
 [core_2_6_1_sdk3]
 platform                  = https://github.com/Jason2866/platform-espressif8266.git#core_2_6_1