Bump asciidoctor from 2.0.20 to 2.0.21

Bumps [asciidoctor](https://github.com/asciidoctor/asciidoctor) from 2.0.20 to 2.0.21.
- [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:development
  update-type: version-update:semver-patch
...

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

Gemfile

@@ -23,7 +23,7 @@ group :development, :lint do
 end
 
 group :development, :docs do
-  gem 'asciidoctor', '~> 2.0', '>= 2.0.20' # for manpage generation
+  gem 'asciidoctor', '~> 2.0' # for manpage generation
   gem 'commonmarker', '~> 0.23' # for markdown support in YARD
   gem 'yard', ['>= 0.9.27', '< 0.10']
 end

Gemfile.lock

@@ -8,7 +8,7 @@ PATH
 GEM
   remote: https://rubygems.org/
   specs:
-    asciidoctor (2.0.20)
+    asciidoctor (2.0.21)
     ast (2.4.2)
     commonmarker (0.23.10)
     docopt (0.6.1)
@@ -48,7 +48,7 @@ PLATFORMS
   x86_64-linux
 
 DEPENDENCIES
-  asciidoctor (~> 2.0, >= 2.0.20)
+  asciidoctor (~> 2.0)
   bundler (~> 2.1)
   commonmarker (~> 0.23)
   docopt (~> 0.6)