Update CircleCI build
Update CircleCI build

- JDK8 build:
  openjdk:8u212-jdk-alpine3.9 -> openjdk:8u232-jdk-slim-buster

- JDK11 build:
  openjdk:11.0.3-jdk-slim-stretch -> openjdk:11.0.5-jdk-slim-buster

- Maven: 3.6.1 -> 3.6.2

marcelmay/greenmail,marcelmay/greenmail,greenmail-mail-test/greenmail,greenmail-mail-test/greenmail,marcelmay/greenmail,marcelmay/greenmail,greenmail-mail-test/greenmail,marcelmay/greenmail