Apple has extended the deadline for the first issue. Instead of April, it is now June 30 for this one:
ITMS-90725: SDK Version Issue - This app was built with the iOS 12.1 SDK. Starting April 2020, all iOS apps submitted to the App Store must be built with the iOS 13 SDK or later, included in Xcode 11 or later.
For the second issue, existing apps will be fine until November 30. New apps will need to use the WKWebView in April:
ITMS-90809: Deprecated API Usage - Apple will stop accepting submissions of apps that use UIWebView APIs starting from April 2020. See https://developer.apple.com/documentation/uikit/uiwebview for more information.