Update asciidoctorj to 3.0.0

Requires overriding the version of the asciidoctor-maven-plugin provided
by the parent POM to 3.0.0

spec/pom.xml

diff --git a/spec/pom.xml b/spec/pom.xml
index 621a9a7..282b37f 100644
--- a/spec/pom.xml
+++ b/spec/pom.xml
@@ -82,7 +82,7 @@
     <properties>
         <site.output.dir>${project.build.directory}/staging</site.output.dir>
         <maven.site.skip>true</maven.site.skip>
-        <asciidoctorj.version>2.5.13</asciidoctorj.version>
+        <asciidoctorj.version>3.0.0</asciidoctorj.version>
         <asciidoctorj.pdf.version>2.3.18</asciidoctorj.pdf.version>
         <jruby.version>9.3.15.0</jruby.version>
         <!-- status: DRAFT, BETA, etc., or blank for final -->
@@ -141,6 +141,8 @@
             <plugin>
                 <groupId>org.asciidoctor</groupId>
                 <artifactId>asciidoctor-maven-plugin</artifactId>
+                <!-- Override parent until parent is updated -->
+                <version>3.0.0</version>
                 <dependencies>
                     <dependency>
                         <groupId>org.jruby</groupId>