Update Spring Framework to 5.2.x / Spring Boot to 2.2.x

According to official roadmap [1], the Spring Framework 5.1.x and 5.0.x are not actively supported anymore, superseded by 5.2.x and 5.3.x as of December 2020. The same applies to Spring Boot 2.1.x (superseded by 2.2.x). For CXF's 3.3.x release line, we probably should update to Spring Framework 5.2.x / Spring Boot to 2.2.x in order to stick with supported versions.

[1] https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Versions