Implement update_group()

[fschrempf]

• Create isolated commit
• Draft test case
• Adjust test case to demo system

[bensteUEM]

@fschrempf - FYI i will change from keyword id to param group_id for consistency
also added some hint for the testcase that a specific permission is required :)

{
"message": "Forbidden to edit the field note",
"translatedMessage": "Keine Berechtigung, das Feld note zu sehen oder zu bearbeiten",
"messageKey": "error.forbidden.edit.field",
"args": {
"field": "note"
},
"errors": []