Deprecate ScreenshotImageFormat configuration for screenshots

[YevgeniyShunevych]

Due to [dotnet] Deprecate saving screenshots with different image formats change in Selenium.WebDriver, Atata will also only support the default PNG format. ScreenshotImageFormat should not be used anymore.

The following screenshots configuration method becomes obsolete and will be removed together with the related functionality in Atata v3:

public static ScreenshotConsumerAtataContextBuilder<TConsumer> With<TConsumer>(
    this ScreenshotConsumerAtataContextBuilder<TConsumer> builder,
    ScreenshotImageFormat imageFormat);