Upgrade plugins (openhab#18042)

* maven-site-plugin to 3.21.0
* maven-plugin-plugin to 3.15.1

Signed-off-by: Holger Friedrich <mail@holger-friedrich.de>

Author: holgerfriedrich

pom.xml

@@ -378,7 +378,7 @@ Import-Package: \\
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-plugin-plugin</artifactId>
-          <version>3.12.0</version>
+          <version>3.15.1</version>
         </plugin>
 
         <plugin>
@@ -399,7 +399,7 @@ Import-Package: \\
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-site-plugin</artifactId>
-          <version>3.12.1</version>
+          <version>3.21.0</version>
         </plugin>
 
         <plugin>