Update Rust crate bytemuck to v1.21.0 (#22873)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [bytemuck](https://redirect.github.com/Lokathor/bytemuck) |
dependencies | minor | `1.20.0` -> `1.21.0` |

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### Release Notes

<details>
<summary>Lokathor/bytemuck (bytemuck)</summary>

###
[`v1.21.0`](https://redirect.github.com/Lokathor/bytemuck/compare/v1.20.0...v1.21.0)

[Compare
Source](https://redirect.github.com/Lokathor/bytemuck/compare/v1.20.0...v1.21.0)

</details>

---

### Configuration

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America/New_York, Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you
are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the
rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update
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---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
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Release Notes:

- N/A

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