Update SNAPSHOT version to v2.21.0-SNAPSHOT

Author: github-actions[bot]

pom.xml

@@ -20,7 +20,7 @@
     </parent>
 
     <artifactId>powsybl-ws-dependencies</artifactId>
-    <version>2.20.0</version>
+    <version>2.21.0-SNAPSHOT</version>
     <packaging>pom</packaging>
 
     <name>PowSyBl Web Services Dependencies</name>