Update asciidoctor requirement from 2.0.20 to 2.0.21

Updates the requirements on [asciidoctor](https://github.com/asciidoctor/asciidoctor) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor@v2.0.20...v2.0.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 gem 'rake', '13.1.0'
-gem 'asciidoctor', '2.0.20'
+gem 'asciidoctor', '2.0.21'
 
 gem 'json', '2.6.3'
 gem 'awesome_print', '1.9.2'