chore: add security reporting banner to the PR template

[dougch]

Goal

Discourage direct security reporting via the pull request process.

Why

This is beyond the scope of a PR why section.

How

Provide contributors with a link to the security reporting page.

Related

https://github.com/aws/s2n-tls/pull/2167

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.