chore(deps): update dependency org.apache.maven.plugins:maven-site-plugin to v3.21.0 (#3834)

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

Author: renovate[bot]

spring-boot-admin-docs/pom.xml

@@ -42,7 +42,7 @@
                     Due to changes in Doxia (see https://cwiki.apache.org/confluence/display/MAVEN/Towards+Doxia+2.0.0+Stack),
                     we have to use a fixed version here, as our site skin does not work with the latest version.
                 -->
-                <version>3.12.1</version>
+                <version>3.21.0</version>
                 <executions>
                     <execution>
                         <id>output-html</id>