deps: update dependency antsibull-docs to >=2.8,<2.9

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
antsibull-docs (changelog)	>=2.7,<2.8 -> >=2.8,<2.9

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Release Notes

ansible-community/antsibull-docs (antsibull-docs)

### [`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 ([#​253](https://togithub.com/ansible-community/antsibull-docs/pull/253), [#​258](https://togithub.com/ansible-community/antsibull-docs/pull/258)). - Add support for the latest antsibull-core v3 pre-release, 3.0.0a1 ([#​250](https://togithub.com/ansible-community/antsibull-docs/pull/250)). - Declare support for Python 3.12 ([#​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 ([#​254](https://togithub.com/ansible-community/antsibull-docs/pull/254)). ##### Bugfixes - Fix duplicate docs detection (for aliases) for latest ansible-core devel ([#​257](https://togithub.com/ansible-community/antsibull-docs/pull/257)).

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