Update security contact and vulnerability disclosure info

CONTRIBUTING.md

@@ -25,7 +25,7 @@ As of July 2018, all the documentation for contributing to the GitLab project ha
 ## Security vulnerability disclosure
-[View the documentation](doc/development/contributing/index.md#security-vulnerability-disclosure) to find the latest information.
+[View the documentation](https://about.gitlab.com/security/disclosure/) to find the latest information.
 ## Code of Conduct

security.txt

-# Preferred disclosure is via HackerOne
-Contact: https://hackerone.com/gitlab/
-# Additional disclosure processes are available in our handbook:
-Contact: https://about.gitlab.com/security/disclosure/
-
-Policy: https://hackerone.com/gitlab/
-
-Acknowledgments: https://hackerone.com/gitlab/thanks
-Acknowledgments: https://about.gitlab.com/security/vulnerability-acknowledgements/
-
-Canonical: https://gitlab.com/gitlab-org/gitlab/-/blob/master/security.txt
-
-Preferred-Languages: en
-Hiring: https://about.gitlab.com/jobs/
-
-Expires: 2024-12-31T00:00:00Z
-
-# This file uses the format described at https://securitytxt.org/