Fix Project Overview entity

[KochTobi]

The project overview entity idicated to JPA a confusing setup with a column and join table for an element collection on the same field. This does always throw exceptions when hibernate is trying to set up the database. Further the wrong '@Immutable` annotation was used.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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