Include inherited owners when calculating User#solo_owned_groups

What does this MR do and why?

Previously User#solo_owned_groups did not calculate with inherited ownership. This could lead to situations where a member would incorrectly destroy a sub-group. This MR includes inheritance in the ownership calculation by counting owners in self and ancestor namespaces.

How to set up and validate locally

1. user1 creates a top-level group
2. user2 adds user2 to the top-level group
3. user2 creates a sub-group within this top-level group. This makes user2 the sole explicit owner of the group, however user1 is an inherited owner from the top-level group.
4. user1 or someone else deletes user2 along with contributions.

Previously: Observe that the sub-group no longer exists.

After the fix: Observe that the sub-group continues to exist, because user1 is an inherited owner from the top level group, so there is no need to delete this subgroup..

Query plan

Typical case

Time: 4.781 ms  
  - planning: 1.138 ms  
  - execution: 3.643 ms  
    - I/O read: 0.000 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 1678 (~13.10 MiB) from the buffer pool  
  - reads: 0 from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

Details and visualization: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/10246/commands/36377.

Worst case

Time: 28.288 ms  
  - planning: 1.396 ms  
  - execution: 26.892 ms  
    - I/O read: 26.261 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 12 (~96.00 KiB) from the buffer pool  
  - reads: 21 (~168.00 KiB) from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

Details and visualization: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/10246/commands/36378.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #26120 (closed)