Updated collect_all_artifacts.sh for Ubuntu Xenial

kokoro/release/collect_all_artifacts.sh

@@ -49,7 +49,7 @@ cp ${INPUT_ARTIFACTS_DIR}/build64/src/protoc protoc/macosx_x64/protoc
 # Install nuget (will also install  mono)
 # TODO(jtattermusch): use "mono:5.14" docker image instead so we don't have to apt-get install
 sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-echo "deb https://download.mono-project.com/repo/ubuntu stable-trusty main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
+echo "deb https://download.mono-project.com/repo/ubuntu stable-xenial main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
 sudo apt update
 sudo apt-get install -y nuget