deps: update module github.com/moby/buildkit to v0.21.1 (#946)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [github.com/moby/buildkit](https://redirect.github.com/moby/buildkit)
| `v0.21.0` -> `v0.21.1` |
[![age](https://developer.mend.io/api/mc/badges/age/go/github.com%2fmoby%2fbuildkit/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/go/github.com%2fmoby%2fbuildkit/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/go/github.com%2fmoby%2fbuildkit/v0.21.0/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/go/github.com%2fmoby%2fbuildkit/v0.21.0/v0.21.1?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>moby/buildkit (github.com/moby/buildkit)</summary>

###
[`v0.21.1`](https://redirect.github.com/moby/buildkit/compare/v0.21.0...v0.21.1)

[Compare
Source](https://redirect.github.com/moby/buildkit/compare/v0.21.0...v0.21.1)

</details>

---

### Configuration

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---

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