Update latest patch version to 10.11.15 (#36090)

Co-authored-by: unified-ci-app[bot] <121569378+unified-ci-app[bot]@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/version.go

@@ -13,6 +13,7 @@ import (
 // It should be maintained in chronological order with most current
 // release at the front of the list.
 var versions = []string{
+	"10.11.15",
 	"10.11.14",
 	"10.11.13",
 	"10.11.12",