Use linear version GroupsWithTemplatesFinder#extended_group_search

What does this MR do and why?

In this MR, we're switching the behavior of two methods in GroupsWithTemplatesFinder#extended_group_search to use the linear version. The new behavior is behind the linear_groups_template_finder_extended_group_search feature flag.

MR acceptance checklist

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Related to #339439 (closed)