Update CLA instructions in issue template

[justaugustus]

@justaugustus the PR template is out of date. Please use https://github.com/rabbitmq/cla.

As for opensource [at] broadcom, we don't yet know how legal would prefer us to collect the signed documents, so I've specified our team's address that is publicly available (Broadcom mailing lists are often not, sadly).

We will pass the signed document to legal one way or another.

Originally posted by @michaelklishin in #14305 (comment)

[michaelklishin]

@justaugustus thank you for bringing this up. I have updated the template and CONTRIBUTING.md.