Limit number of group memberships to be managed with "add" step to 10

[kwin]

In fact the limit seems to be 20 but documented is 10, so better be conservative here. Compare with https://adobe-apiplatform.github.io/umapi-documentation/en/api/ActionsCmds.html

This closes #753

[sonarcloud[bot]]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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