NIFI-15487 Updated version to 2.6.0

Author: pvillard31

pom.xml

@@ -10,7 +10,7 @@
 
     <groupId>org.apache.nifi</groupId>
     <artifactId>nifi-api</artifactId>
-    <version>2.6.0-SNAPSHOT</version>
+    <version>2.6.0</version>
     <packaging>jar</packaging>
 
     <name>nifi-api</name>