asciidoctor: update to v2.0.21

TESTED:
Tested on 10.4-10.5 ppc, 10.4-10.6 i386, 10.5-12.x x86_64, and
11.x-14.x arm64.
Builds and passes tests on all versions.
Functionally tested with gpsd and ntpsec on 10.9.  Generated docs
match except for expected metadata differences specifically related to
asciidoctor version.

textproc/asciidoctor/Portfile

@@ -7,7 +7,7 @@ PortGroup           ruby 1.0
 # architectures.  Don't change it without preserving this property.
 # The choice of Ruby versions doesn't affect clients.
 #
-ruby.setup          asciidoctor 2.0.20 gem {} rubygems ruby30
+ruby.setup          asciidoctor 2.0.21 gem {} rubygems ruby30
 name                asciidoctor
 revision            0
 
@@ -32,9 +32,9 @@ long_description    Asciidoctor is a fast, open source, Ruby-based \
                     content written in AsciiDoc.
 homepage            https://asciidoctor.org/
 
-checksums           rmd160  e75d63c088cfba353283b93a314f774386e1080a \
-                    sha256  835eabd445e4ae88f56a5f4e07593c3612b2be72eb661c612c3a8e1e17c57479 \
-                    size    281600
+checksums           rmd160  9e45eab40c558c947a55697c8362ea4414f04bac \
+                    sha256  07138c69d2aa320932d38beb17fedb09090cdb38e64d14c1f6926b620715c100 \
+                    size    282624
 
 # Use normal distfile location, instead of 'ruby'.
 dist_subdir         ${name}