chore(deps): update dependency org.asciidoctor:asciidoctorj-pdf to v2.3.21 (#2106)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

boot-rest-docs-sample/pom.xml

@@ -191,7 +191,7 @@
                     <dependency>
                         <groupId>org.asciidoctor</groupId>
                         <artifactId>asciidoctorj-pdf</artifactId>
-                        <version>2.3.20</version>
+                        <version>2.3.21</version>
                     </dependency>
                 </dependencies>
             </plugin>