[21309] CI: paths-ignore only ignores modification in markdown files (backport #876)

[mergify]

Description

This PR enables requesting CI on PRs that only modify .txt files, such as:

• [21579] Bump setuptools from 67.6.0 to 70.0.0 in /docs #872

@Mergifyio backport 2.14.x 2.10.x 2.6.x

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A: Code snippets related to the added documentation have been provided.
• N/A: Documentation tests pass locally.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

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This is an automatic backport of pull request #876 done by [Mergify](https://mergify.com).