Updates security policy with CVD

[vmoutoussamy]

1. We need to update our security policy and reference our newly published CVD policy: https://www.suse.com/support/security/cvd-policy/
2. I took the liberty to shorten the text and try to make it a bit more readable
3. New email for reporting: psirt@suse.com and new GPG Public key

[vmoutoussamy]

@olblak or @holyspectral could you please review so we can merge this PR? CNS doesn't qualify as "reviewers with write access".