2.1.261

Update asciidoctor-epub3 requirement from 1.5.1 to 2.1.0 (#368)

Updates the requirements on [asciidoctor-epub3](https://github.com/asciidoctor/asciidoctor-epub3) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-epub3/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-epub3/blob/main/CHANGELOG.adoc)
- [Commits](https://github.com/asciidoctor/asciidoctor-epub3/compare/v1.5.1...v2.1.0)

---
updated-dependencies:
- dependency-name: asciidoctor-epub3
  dependency-type: direct:production
...

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