Update Groups & Projects category

Description

This updates the category from groups_&_projects to groups_and_projects. Follow-up from !123977 (merged).

While working on the GitLab project to update the feature_category after merging Category:Subgroups and Category:Projects to Category:Groups & Projects, I'm getting Rubocop errors because the current category (groups_&_projects) is invalid as Rubocop assumes & as an operator, and it's an unusual variable name.

Related to #14554 (closed).