deps: update dependency antsibull-docs to >=2.6,<2.7 (#414)

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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)
| `>=2.5.0,<2.6` -> `>=2.6,<2.7` |
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|
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|
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|
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|

---

### Release Notes

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

###
[`v2.6.0`](https://togithub.com/ansible-community/antsibull-docs/blob/HEAD/CHANGELOG.rst#v260)

[Compare
Source](https://togithub.com/ansible-community/antsibull-docs/compare/2.5.0...2.6.0)

\======

## Release Summary

Fix parsing of `EXAMPLES` and improve error message

## Minor Changes

- Improve error messages when calls to `ansible-doc` fail
([https://github.com/ansible-community/antsibull-docs/pull/223](https://togithub.com/ansible-community/antsibull-docs/pull/223)).

## Bugfixes

- When `EXAMPLES` has the format specified by `# fmt: <format>`, this
value is used to determine the code block type
([https://github.com/ansible-community/antsibull-docs/pull/225](https://togithub.com/ansible-community/antsibull-docs/pull/225)).

</details>

---

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requirements.txt

@@ -9,4 +9,4 @@ requests
 
 # Development requirements
 pylint
-antsibull-docs>=2.5.0,<2.6
+antsibull-docs>=2.6,<2.7