Update rest-backend and rest-lib-utils to SpringBoot 2.5.1

[serenaponta]

Update spring services from version 1.5.20 to 2.1.4

[serenaponta]

We cannot use a migration to fix the checksum as (of course) spring/flyway will not do anything (apply new migrations or create a baseline) if the validation does not go through.

I think the checksum issue may be caused by the fact that the latest flyway release does not support Postgres 11. They added support in flyway 6.0 but it only exists as a beta. On top of the fact that it's not recommended to use a beta in production, we cannot even test it as we get the same error of as "Spring Boot is not yet compatible with Flyway 6.0". As far as they say it will be once they release 6.0 (no beta).

As it does not make sense to downgrade postgres, i think we need to wait for the release of 6.0 to finally move to spring2.

[serenaponta]

In Spring2 the default for Id generator changed:
"The spring.jpa.hibernate.use-new-id-generator-mappings property is now true by default to align with the default behaviour of Hibernate. If you need to temporarily restore this now deprecated behaviour, set the property to false."

The new default was causing our test to run awfully slow (e.g. testGetAppVulnerabilitiesForBundledLibs was running for 14 minutes instead of 2), spending most of the time in the saving of the libraries whose debug showed calls to an hibernate sequence to get the next value.

For the time being we reverted to the configuration used in Spring 1.5 (false), we will need to investigate if the new default also causes conflicts with the existing data and how to migrate.

[henrikplate]

There are some unresolved merge conflicts in license headers + JavaDoc, and I suggest to delete some stuff rather than commenting it out

[serenaponta]

Review comments are addressed

[henrikplate]

Well done 👍