Safe templating for backport and copy titles & bodies

The title and body of the copy and backport actions now render with Mergify’s safe message-templating engine: literal text plus allowlisted {{ variable }} placeholders, with no expression evaluation. Plain {{ variable }} placeholders keep working, but richer Jinja2 in these fields — filters, loops, and {% … %} logic (for example {{ title | upper }} or {% for … %}) — is now deprecated and will stop working in a future release.