fix(deps): update rust crate schemars to v0.8.21

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
schemars (source)	dependencies	patch	0.8.20 -> 0.8.21

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Release Notes

GREsau/schemars (schemars)

### [`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 ([https://github.com/GREsau/schemars/issues/295](https://togithub.com/GREsau/schemars/issues/295) / [https://github.com/GREsau/schemars/pull/296](https://togithub.com/GREsau/schemars/pull/296))

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