[maven-release-plugin] prepare branch 0.20

pom.xml

@@ -17,7 +17,7 @@
   <scm>
       <connection>scm:git:https://github.com/dCache/nfs4j.git</connection>
       <developerConnection>scm:git:ssh://git@github.com/dCache/nfs4j.git</developerConnection>
-    <tag>HEAD</tag>
+    <tag>0.20</tag>
   </scm>
 
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