chore(deps): update dependency antsibull-docs to >=2.20,<2.21 (#672)

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))
| `>=2.19,<2.20` -> `>=2.20,<2.21` |
[![age](https://developer.mend.io/api/mc/badges/age/pypi/antsibull-docs/2.20.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/antsibull-docs/2.19.1/2.20.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

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

###
[`v2.20.0`](https://redirect.github.com/ansible-community/antsibull-docs/releases/tag/2.20.0)

[Compare
Source](https://redirect.github.com/ansible-community/antsibull-docs/compare/2.19.1...2.20.0)

##### Release Summary

Bugfix and feature release.

##### Minor Changes

- Add <code>--config</code> parameter to the <code>ansible-output</code>
subcommand to allow specifying a config file when not in collection mode
([#&#8203;410](https://redirect.github.com/ansible-community/antsibull-docs/pull/410)).
- Add <code>ansible-output-meta</code> directive that allows to apply
meta actions, like resetting previous code blocks for variable
references, or defining templates for <code>ansible-output-data</code>
([#&#8203;409](https://redirect.github.com/ansible-community/antsibull-docs/pull/409)).
- Allow to specify YAML inventory for <code>ansible-output</code>
subcommand code blocks
([#&#8203;411](https://redirect.github.com/ansible-community/antsibull-docs/pull/411)).
- Let <code>ansible-output</code> subcommand run
<code>ansible-playbook</code> instances in parallel
([#&#8203;407](https://redirect.github.com/ansible-community/antsibull-docs/pull/407),
[#&#8203;408](https://redirect.github.com/ansible-community/antsibull-docs/pull/408)).

##### Bugfixes

- Run the post-processors in the <code>ansible-output</code> subcommand
from the current working directory, and not from the temporary playbook
directory. This allows post-processors to have relative paths from where
<code>antsibull-docs ansible-output</code> is called
([#&#8203;406](https://redirect.github.com/ansible-community/antsibull-docs/pull/406)).

</details>

---

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---

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---

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requirements-dev.txt

@@ -6,4 +6,4 @@ cryptography
 
 # Development requirements
 pylint
-antsibull-docs>=2.19,<2.20
+antsibull-docs>=2.20,<2.21