deps: update dependency antsibull-docs to >=2.8,<2.9 (#471)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
|
[antsibull-docs](https://togithub.com/ansible-community/antsibull-docs)
([changelog](https://togithub.com/ansible-community/antsibull-docs/tree/main/CHANGELOG.md))
| `>=2.7,<2.8` -> `>=2.8,<2.9` |
[![age](https://developer.mend.io/api/mc/badges/age/pypi/antsibull-docs/2.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/pypi/antsibull-docs/2.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/pypi/antsibull-docs/2.7.0/2.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
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|

---

### Release Notes

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

###
[`v2.8.0`](https://togithub.com/ansible-community/antsibull-docs/releases/tag/2.8.0)

[Compare
Source](https://togithub.com/ansible-community/antsibull-docs/compare/2.7.0...2.8.0)

##### Release Summary

Bugfix and feature release.

##### Minor Changes

- Add support for "dark mode" to the option table styling
([#&#8203;253](https://togithub.com/ansible-community/antsibull-docs/pull/253),
[#&#8203;258](https://togithub.com/ansible-community/antsibull-docs/pull/258)).
- Add support for the latest antsibull-core v3 pre-release,
<code>3.0.0a1</code>
([#&#8203;250](https://togithub.com/ansible-community/antsibull-docs/pull/250)).
- Declare support for Python 3.12
([#&#8203;255](https://togithub.com/ansible-community/antsibull-docs/pull/255)).
- The colors used by the CSS provided by the Antsibull Sphinx extension
can now be overridden
([#&#8203;254](https://togithub.com/ansible-community/antsibull-docs/pull/254)).

##### Bugfixes

- Fix duplicate docs detection (for aliases) for latest ansible-core
devel
([#&#8203;257](https://togithub.com/ansible-community/antsibull-docs/pull/257)).

</details>

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requirements.txt

@@ -9,4 +9,4 @@ requests
 
 # Development requirements
 pylint
-antsibull-docs>=2.7,<2.8
+antsibull-docs>=2.8,<2.9