Bump league/commonmark from 2.3.1 to 2.6.0

Bumps [league/commonmark](https://github.com/thephpleague/commonmark) from 2.3.1 to 2.6.0.
- [Release notes](https://github.com/thephpleague/commonmark/releases)
- [Changelog](https://github.com/thephpleague/commonmark/blob/2.6/CHANGELOG.md)
- [Commits](thephpleague/commonmark@2.3.1...2.6.0)

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updated-dependencies:
- dependency-name: league/commonmark
  dependency-type: indirect
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Author: dependabot[bot]