Updated Flutter driver to version 2.15.0

Author: budtmo

Appium/Dockerfile

@@ -148,7 +148,7 @@ USER 1300:1301
 # Install basic Android drivers
 #===============================
 ENV APPIUM_DRIVER_ESPRESSO_VERSION="4.1.5"
-ENV APPIUM_DRIVER_FLUTTER_VERSION="2.13.0"
+ENV APPIUM_DRIVER_FLUTTER_VERSION="2.15.0"
 ENV APPIUM_DRIVER_FLUTTER_INTEGRATION_VERSION="1.1.3"
 ENV APPIUM_DRIVER_GECKO_VERSION="1.4.2"
 ENV APPIUM_DRIVER_UIAUTOMATOR2_VERSION="4.0.2"