Update dependency org.apache.maven.plugins:maven-site-plugin to v3.21.0 (#401)

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

Author: renovate[bot]

pom.xml

@@ -206,7 +206,7 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-site-plugin</artifactId>
-				<version>3.20.0</version>
+				<version>3.21.0</version>
 				<configuration>
 					<locales>de</locales>
 				</configuration>