[deps]: Update Rust crate schemars to v0.8.21 (#801)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [schemars](https://graham.cool/schemars/)
([source](https://togithub.com/GREsau/schemars)) | dependencies | patch
| `0.8.19` -> `0.8.21` |

---

### Release Notes

<details>
<summary>GREsau/schemars (schemars)</summary>

###
[`v0.8.21`](https://togithub.com/GREsau/schemars/blob/HEAD/CHANGELOG.md#0821---2024-05-23)

[Compare
Source](https://togithub.com/GREsau/schemars/compare/v0.8.20...v0.8.21)

##### Fixed:

- Fix `null` default not being set on generated schemas
([GREsau/schemars#295
/
[GREsau/schemars#296)

###
[`v0.8.20`](https://togithub.com/GREsau/schemars/blob/HEAD/CHANGELOG.md#0820---2024-05-18)

[Compare
Source](https://togithub.com/GREsau/schemars/compare/v0.8.19...v0.8.20)

##### Fixed:

- Revert unintentional change in behaviour when combining `default` and
`required` attributes
([GREsau/schemars#292)

</details>

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