Use parent 51

Parent 50 is affected by SLING-11944, releases can't be performed.
diff --git a/pom.xml b/pom.xml
index 1c206dc..b8393f3 100644
--- a/pom.xml
+++ b/pom.xml
@@ -23,7 +23,7 @@
     <parent>
         <groupId>org.apache.sling</groupId>
         <artifactId>sling</artifactId>
-        <version>50</version>
+        <version>51</version>
         <relativePath />
     </parent>