bump maven-site-plugin from 3.9.1 to 3.12.1 (via #268)

Author: dependabot[bot]

pom.xml

@@ -105,7 +105,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-site-plugin</artifactId>
-                    <version>3.9.1</version>
+                    <version>3.12.1</version>
                 </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>