Add Deprecation/Removal Announcement for ROPC without Client Credentials on GitLab.com

ROPC without client credentials was removed on GitLab.com on April 08, 2025 due to security concerns - https://about.gitlab.com/blog/2025/04/01/improving-oauth-ropc-security-on-gitlab-com/

We should capture this in a removal announcement, so we can make sure we have this covered in our standard removal processes.