Re-use field validators if `pydantic v2` permits

Review comment: https://github.com/kernelci/kernelci-api/pull/572#discussion_r1860429614
We have the same field validator definitions for `groups` field in user schema models such as `UserCreate` and `UserUpdate`.
Check if field validators can be reused in pydantic v2 and optimize models if possible.

Start with the UserCreate and UserUpdate schema models and the linked pull-request review discussion. Check how their `groups` field validators are defined and whether Pydantic v2 supports reusing them; the work is done when duplicate definitions are consolidated without changing validation behavior.

Written by the indexing model from the issue text.