Adjust finder to scope by group

What does this MR do and why?

Turns out that namespace: group isn't a real argument in ProjectsFinder and what I really wanted was GroupProjectsFinder.

Adjust finder to scope by group.

Database query (e.g. for group 9970):

SELECT "projects".* FROM ((SELECT "projects".* FROM ((SELECT "projects".* FROM "projects" WHERE "projects"."namespace_id" IN (SELECT "namespaces"."id" FROM "namespaces" WHERE "namespaces"."type" = 'Group' AND (traversal_ids @> ('{9970}'))))) projects WHERE (EXISTS (SELECT 1 FROM "project_authorizations" WHERE "project_authorizations"."user_id" = 1 AND (project_authorizations.project_id = projects.id)) OR projects.visibility_level IN (0,10,20)))
UNION
(SELECT "projects".* FROM "projects" INNER JOIN "project_group_links" ON "projects"."id" = "project_group_links"."project_id" WHERE "project_group_links"."group_id" = 9970 AND (EXISTS (SELECT 1 FROM "project_authorizations" WHERE "project_authorizations"."user_id" = 1 AND (project_authorizations.project_id = projects.id)) OR projects.visibility_level IN (0,10,20)))) projects WHERE "projects"."pending_delete" = FALSE AND "projects"."hidden" = FALSE ORDER BY "projects"."id" DESC

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/19716/commands/64561

Screenshots or screen recordings

See !120979 (merged) for GraphQL query and local testing steps.

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Related to #411537 (closed)