Draft: Enable linear version of `User#manageable_groups`
What does this MR do and why?
In this MR, we're switching the behavior of two methods in User#manageable_groups
to use the linear version. The new behavior is behind the linear_user_manageable_groups
feature flag.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #339434 (closed)