NAID-2571: Update mockito-core to 5.15.2

[MartinWheelerMT]

What

• Update mockito-core to 5.15.2
• Update deep mocks in QuestionnaireMapperTest to return null for user mock due to changes in the way mocks are handled between versions.

Why

To ensure dependency are up to date.

Type of change

Please delete options that are not relevant.

• Internal change (non-breaking change with no effect on the functionality affecting end users)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation where required
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change affects end users.

[github-actions]

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