chore(deps): downgrade maven-site-plugin version to 3.21.0

Downgrade necessary as site plugin v4+ is not compatible with Maven 3 (which we use everywhere).

Author: poikilotherm

pom.xml

@@ -164,7 +164,7 @@
         <maven-failsafe-plugin.version>3.5.4</maven-failsafe-plugin.version>
         <maven-assembly-plugin.version>3.3.0</maven-assembly-plugin.version>
         <maven-plugin-plugin.version>3.15.2</maven-plugin-plugin.version>
-        <maven-site-plugin.version>4.0.0-M16</maven-site-plugin.version>
+        <maven-site-plugin.version>3.21.0</maven-site-plugin.version>
         <maven-source-plugin.version>3.4.0</maven-source-plugin.version>
         <maven-javadoc-plugin.version>3.12.0</maven-javadoc-plugin.version>
         <maven-flatten-plugin.version>1.7.3</maven-flatten-plugin.version>