Exclude project namespaces from autocomplete routes finder

What does this MR do and why?

Excludes project namespaces from group autocomplete routes finder.

Related to #341823 (closed)

DB query

warm cache: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/7022/commands/24860

SELECT "routes".* FROM "routes" WHERE "routes"."source_type" = 'Namespace' AND "routes"."source_id" IN (SELECT "namespaces"."id" FROM "namespaces" WHERE (type IS DISTINCT FROM 'Project')) AND "routes"."path" ILIKE '%gitlab%' ORDER BY LENGTH(routes.path), "routes"."path" ASC LIMIT 20

Time: 2.493 s  
  - planning: 1.819 ms  
  - execution: 2.492 s  
    - I/O read: 947.860 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 188038 (~1.40 GiB) from the buffer pool  
  - reads: 27249 (~212.90 MiB) from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

This seems to be slow, though the old query is slow too (warm cache): https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/7022/commands/24858

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