Azure DevOps Build Xamarin.IOS with IOS 12 as target

You should be able to select the correct version of Mono and Xcode now based on this documentation. In Azure DevOps in both the build and the release pipelines (before the Xamarin.iOS build task and the release task respectively), I added a command line task that runs the following:

echo 'Updating fastlane...'
sudo gem install fastlane

echo 'Selecting Mono version...'
/bin/bash -c "sudo $AGENT_HOMEDIRECTORY/scripts/select-xamarin-sdk.sh 5_16_0"

echo 'Selecting Xcode version...'
/bin/bash -c "echo '##vso[task.setvariable variable=MD_APPLE_SDK_ROOT;]'/Applications/Xcode_10.1.app;sudo xcode-select --switch /Applications/Xcode_10.1.app/Contents/Developer"