Fix dependency warnings in Markdown module

[INFO] --- maven-dependency-plugin:3.4.0:analyze (default-cli) @ doxia-module-markdown ---
[WARNING] Used undeclared dependencies found:
[WARNING]    org.slf4j:slf4j-api:jar:1.7.36:compile
[WARNING]    com.vladsch.flexmark:flexmark-util-data:jar:0.62.2:compile
[WARNING]    com.vladsch.flexmark:flexmark-util-misc:jar:0.62.2:compile
[WARNING]    org.opentest4j:opentest4j:jar:1.2.0:test
[WARNING]    com.vladsch.flexmark:flexmark-util-sequence:jar:0.62.2:compile
[WARNING]    com.vladsch.flexmark:flexmark-util-ast:jar:0.62.2:compile
[WARNING] Unused declared dependencies found:
[WARNING]    com.vladsch.flexmark:flexmark-util:jar:0.62.2:compile
[WARNING]    com.google.inject:guice:jar:no_aop:4.2.3:test
[WARNING]    org.slf4j:slf4j-simple:jar:1.7.36:test
[WARNING]    org.hamcrest:hamcrest-core:jar:2.2:test
[WARNING]    org.junit.jupiter:junit-jupiter-engine:jar:5.9.2:test