[6.x] Fix Eloquent user merge setting roles and groups as model attributes

[duncanmcclean]

This pull request fixes an issue where calling merge() on an Eloquent user would cause a Column not found SQL error for roles and groups.

This was happening because merge() round-trips through data() — the getter injects roles and groups from their relationship tables, but the setter passes every key through set(), which assigns directly to the Eloquent model. When the model saves, it tries to write roles and groups columns that don't exist.

This PR fixes it by stripping roles and groups from the data before passing it back through the setter, since they're managed via their own relationship tables and should never be set as model attributes.

Fixes duncanmcclean/statamic-cargo#176