Add scope to exclude project namespaces

What does this MR do and why?

There will be multiple places where we will need to exclude project namespaces (see #341823 (comment 697264917)) so let's add a scope for it so we can work independently on addressing these places.

DB query

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/6813/commands/24073

select count(*) from namespaces where type IS DISTINCT FROM 'Project';

Time: 3.438 s
  - planning: 0.288 ms
  - execution: 3.438 s
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 10594684 (~80.80 GiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

This is with warm cache.

#341823 (closed)

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