Upgrade to spring-boot 2.1.x with hibernate 5.3 and JPA 2.2

[hohwille]

Hibernate 5.3 is already out since a while and fully supports JPA 2.2:
https://www.thoughts-on-java.org/hibernate-5-3/
We should therefore use the official JPA 2.2 by replacing

      <dependency>
        <groupId>org.hibernate.javax.persistence</groupId>
        <artifactId>hibernate-jpa-2.1-api</artifactId>
        <version>1.0.0.Final</version>
      </dependency>

with

      <dependency>
        <groupId>javax.persistence</groupId>
        <artifactId>javax.persistence-api</artifactId>
        <version>2.2</version>
      </dependency>

To do so we also need to upgrade to hibernate 5.3 what will come with spring-boot 2.1.0:
spring-projects/spring-boot#11725
But currently not even the milestone 2.1.0.M1 is in maven central.