fix: Include free plans for api use notifications

[zachaysan]

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• [x] I have run pre-commit to check linting
• [x] I have added information to docs/ if required so people know about the feature!
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Changes

Includes free plans in the API use notifications with a rolling 30 day window as their effective start date. It also included some extra code to ensure that they're not including in automatic payment adjustments since free plans don't have credit cards.

How did you test this code?

Two existing tests and a new one just for free plans.

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3 Ignored Deployments

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/docs/GKEULKisEL9g6TNrmtgzSqUuyX5s)) | [Visit Preview](https://docs-git-fix-includefreeplansforapiusenotifications-flagsmith.vercel.app) | | Jun 20, 2024 6:43pm | | **flagsmith-frontend-preview** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-preview/HzU8FHVRoG5ko9RDFpiNDgEBkC8N)) | [Visit Preview](https://flagsmith-frontend-preview-git-fix-includefree-d95c52-flagsmith.vercel.app) | | Jun 20, 2024 6:43pm | | **flagsmith-frontend-staging** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-staging/9LXiBWCR5pdx3hHu6YZvuUdD7bYg)) | [Visit Preview](https://flagsmith-frontend-staging-git-fix-includefree-283ce8-flagsmith.vercel.app) | | Jun 20, 2024 6:43pm |

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.51%. Comparing base (e7fd8bd) to head (f491c2f). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4204 +/- ## ======================================= Coverage 96.51% 96.51% ======================================= Files 1177 1177 Lines 38230 38290 +60 ======================================= + Hits 36896 36956 +60 Misses 1334 1334 ```

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