Adding indexes for group collection in session service

[dippindots]

This is the follow up issue of https://github.com/cBioPortal/cbioportal/issues/9757

Proposed solution:

• Change SessionRepositoryImpl.java to create that two indexes for group collection
• Add documentation for existing database migration (because this won’t create index for existing database)
• Add a comment line above the VirtualStudyData class to remind us if we need to change this class, make sure we check the indexes we created in session service.