chore(deps): update dependency antsibull-docs to >=2.21,<2.22 (#678)

This PR contains the following updates:

| Package | Change | Age | Confidence |
|---|---|---|---|
|
[antsibull-docs](https://redirect.github.com/ansible-community/antsibull-docs)
([changelog](https://redirect.github.com/ansible-community/antsibull-docs/tree/main/CHANGELOG.md))
| `>=2.20,<2.21` -> `>=2.21,<2.22` |
[![age](https://developer.mend.io/api/mc/badges/age/pypi/antsibull-docs/2.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/antsibull-docs/2.20.0/2.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>ansible-community/antsibull-docs (antsibull-docs)</summary>

###
[`v2.21.0`](https://redirect.github.com/ansible-community/antsibull-docs/releases/tag/2.21.0)

[Compare
Source](https://redirect.github.com/ansible-community/antsibull-docs/compare/2.20.0...2.21.0)

##### Release Summary

Feature release.

##### Minor Changes

- Add an index for all deprecated collections (official docsite only)
and plugins
([#&#8203;413](https://redirect.github.com/ansible-community/antsibull-docs/pull/413)).

</details>

---

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requirements-dev.txt

@@ -6,4 +6,4 @@ cryptography
 
 # Development requirements
 pylint
-antsibull-docs>=2.20,<2.21
+antsibull-docs>=2.21,<2.22