Revert "Bump doxia-module-markdown from 1.9.1 to 1.10"

This reverts commit 864fa0d.

It fails with:

    An API incompatibility was encountered while executing org.apache.maven.plugins:maven-site-plugin:3.9.1:site: java.lang.AbstractMethodError: org.apache.maven.doxia.module.markdown.MarkdownParser.parse(Ljava/io/Reader;Lorg/apache/maven/doxia/sink/Sink;)V

pom.xml

@@ -149,7 +149,7 @@
                     <dependency>
                         <groupId>org.apache.maven.doxia</groupId>
                         <artifactId>doxia-module-markdown</artifactId>
-                        <version>1.10</version>
+                        <version>1.9.1</version>
                     </dependency>
                 </dependencies>
                 <executions>