[Lagrangian.Model] Use base class Data instead of protected class member for constraint id

[alxbilger]

related to https://github.com/SofaDefrost/SoftRobots/pull/283

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• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[alxbilger]

[ci-build][with-all-tests]