Bump MAX_GROUPS to allow for more groups

Addresses #219 (closed). Especially Radius accounts might belong to a large number of groups.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code