bump version to v3.15.8 (#1007)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

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: "3.15.7"
+version: "3.15.8"
 usage: "Preview helm upgrade changes as a diff"
 description: "Preview helm upgrade changes as a diff"
 useTunnel: true