Add workaround for PMD 7.0.0-rc4 and Java 21

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

pom.xml

@@ -3,7 +3,6 @@
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
 
   <modelVersion>4.0.0</modelVersion>
-
   <groupId>org.openhab.tools.sat</groupId>
   <artifactId>pom</artifactId>
   <version>0.16.0-SNAPSHOT</version>
@@ -94,6 +93,11 @@
   </properties>
 
   <dependencies>
+    <dependency>
+      <groupId>net.sourceforge.pmd</groupId>
+      <artifactId>pmd-compat6</artifactId>
+      <version>7.0.0-rc4+with-pr4749</version>
+    </dependency>
     <dependency>
       <groupId>javax.xml.bind</groupId>
       <artifactId>jaxb-api</artifactId>