build: bump version to 3.15.6 (#981)

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Author: Copilot

plugin.yaml

@@ -1,7 +1,7 @@
 name: diff
 # Version is the version of Helm plus the number of official builds for this
 # plugin
-version: "3.15.5"
+version: "3.15.6"
 usage: "Preview helm upgrade changes as a diff"
 description: "Preview helm upgrade changes as a diff"
 useTunnel: true