Use new models in groups plugin

[pcrespov]

In #3352 we introduced groups_models module that includes pydantic models to represent the rest API schemas used in the groups entrypoints.
We should extend its use to the group's plugin logic

• ♻️ Enhanced groups/organizations web-api specs and validation 🚨 #6640: api models
• remove all legacy validation in _utils such as convert_groups_db_to_schema etc ... and replace with internal models. OMG, remove this ASAP!
• Fix inconsistency with MyGroupGet in bewteen GET /me is different as in GET /groups