Fix template name collision between new and legacy frontend plugin templates

The legacy frontend plugin template had the same name (`frontend-plugin`) as
the new frontend plugin template, causing a conflict error when both were shown
(e.g. when the frontend system could not be auto-detected). Rename the legacy
template to `frontend-plugin-legacy` so both can coexist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

packages/cli-module-new/templates/legacy-frontend-plugin/portable-template.yaml

@@ -1,4 +1,4 @@
-name: legacy-frontend-plugin
+name: frontend-plugin-legacy
 role: frontend-plugin
 description: A new frontend plugin (legacy system)
 values: