fix: incorrect statistics in organisation admin list

[matthewelwell]

Changes

Fixes incorrect statistics in organisation admin list view.

How did you test this code?

Manually:

1. Run the API locally
2. Create a project in a given organisation
3. Confirm that the number of projects in the django admin list is correct
4. Delete the project
5. Confirm that the number of projects in the django admin list is correct

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Aug 28, 2024 10:16am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Aug 28, 2024 10:16am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Aug 28, 2024 10:16am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4546	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4546	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4546	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4546	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4546	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4546	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-55640 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.01%. Comparing base (aa234e4) to head (3c1406c).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4546   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files        1152     1152           
  Lines       39680    39678    -2     
=======================================
- Hits        38495    38494    -1     
+ Misses       1185     1184    -1     

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