Remove shared cache mode property

As can be seen in #69 (comment), we currently have, in application.properties, a cache mode property that simply overrides the deprecated setting used by Hibernate:

website-niaefeup-backend/src/main/resources/application.properties

Lines 28 to 30 in c07586e

    
           # Due to a problem with Hibernate, which is using a deprecated property. This should be removed when fixed 
        
           # See https://github.com/spring-projects/spring-data-jpa/issues/2717 for more information 
        
           spring.jpa.properties.jakarta.persistence.sharedCache.mode=UNSPECIFIED

You can read more about this issue here. This property should be removed whenever the problem with Hibernate is fixed.

The problem was fixed and will be updated in Hibernate 6.1.7 (to be released): https://hibernate.atlassian.net/browse/HHH-15768

The hibernate version is now bumped in spring boot, we should update it (and perhaps configure dependabot here)

	# Due to a problem with Hibernate, which is using a deprecated property. This should be removed when fixed
	# See https://github.com/spring-projects/spring-data-jpa/issues/2717 for more information
	spring.jpa.properties.jakarta.persistence.sharedCache.mode=UNSPECIFIED