Release notes on Browserstack

Upgraded Appium 2.0 driver versions supported on App Automate:

• XCUITest (iOS)
• Flutter (Android & iOS)
• UIAutomator2 (Android)

Latest stable Appium 2.0 driver versions supported on App Automate:

• UIAutomator2: 2.14.0
• Flutter: 1.14.3
• XCUITest: 4.21.6

Users can use these latest drivers by passing "latest" flag in the "automationVersion" capability