I think we all are working on a very dynamic and fast paced environment. So I would suggest you get to know the project first in order to then be specific on how you start your learning journey. If the project is using SpringBoot then great because you don't have to do the dirty work of doing manual configuration/xml based confguration. SpringBoot makes it easier to develop, mainly as developers we foucs on business logic.

From spring2.5+, there is a heavy usage of annotations instead of xml. So you can start with spring5 because then getting to springboot would be easier.

You can start with these modules on spring-5: spring-core, web-mvc and spring-data-jpa to begin with.