Upgrade Spring boot & framework

[henrietteharmse]

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Currently OLS4 is using version Spring framework 2.4.5.RELEASE and Spring boot 1.3.8.RELEASE. These versions I think come from OLS3 and we may be forced to use it for backward compatibility of our REST API, but at least we should verify whether we can upgrade to a later version of Spring, if not the latest.

Latest versions available for comparison are 5.3.23 for framework and 2.7.5 for boot.

[henrietteharmse]

If there is a stumbling block it is likely to be HATEOAS. Currently we are using version 0.19.0.RELEASE of org.springframework.hateoas with 2.0.0 being available. Here is the migration guide for reference: https://docs.spring.io/spring-hateoas/docs/current/reference/html/#migrate-to-1.0

[jamesamcl]

We can also remove the thymeleaf dependency.

EDIT: done