Use multiple options by default for `RolesMatrixType` form

[phansys]

Subject

Use multiple options by default for RolesMatrixType form

I am targeting this branch, because this change seems to follow the natural use case for this form, where multiple options for the role matrix are allowed (rendered as checkboxes instead of radio buttons):

Changelog

### Added
- Default value `true` for the "multiple" option at `RolesMatrixType` form type

### Changed
- `RolesMatrixType` form type allows multiple values by default

[VincentLanglet]

Thanks