Fix masked_and_hidden interpretation in Group-level variables API

What does this MR do and why?

This change set fixes the interpretation of the masked_and_hidden field in the Group-level Variables REST API.

Edit: it also fixes the other boolean fields.

💡 Can be reviewed commit-by-commit 🕵️

References

#556872 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.