Maintenance: Switch version to upcoming 5.2.

CHANGELOG.md

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 # Change Log
 
-## [5.1.0](https://github.com/zammad/zammad/tree/5.1.0) (2021-xx-xx)
-[Full Changelog](https://github.com/zammad/zammad/compare/5.0.0...5.1.0)
+## [5.2.0](https://github.com/zammad/zammad/tree/5.2.0) (2022-xx-xx)
+[Full Changelog](https://github.com/zammad/zammad/compare/5.1.0...5.2.0)
 
 **Implemented enhancements:**
 
@@ -9,7 +9,3 @@
 
 
 **Fixed bugs:**
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-
-

VERSION

@@ -1 +1 @@
-5.1.x
+5.2.x