Deprecate old custom CA test tooling

Description

As a last step for &6850, we should clean up old projects. There is a chance that any of the projects in use today to test custom CA certs will be used in the future iteration of testing custom CA certs. Therefore, we should update this issue at the end of &6850.

Tasks

• Check to see if any other groups want to rely on the below projects.
• If No:
  • Deprecate and schedule a removal for: https://gitlab.com/gitlab-org/security-products/tests/custom-ca
  • Deprecate and schedule a removal for: https://gitlab.com/gitlab-org/security-products/tests/custom-ca-ssl-server