Pushing out the removal to 19.0 as we forgot about an important feature to offer an alternative for

What does this MR do and why?

It moves the removal of the certificate-based cluster integration to 19.0 following user feedback on missing features for GitLab-managed namespace creation.

Impact evaluation based on https://gitlab-com.gitlab.io/gl-infra/breaking-change-impact-calculator/?usage=adopted&migration_complexity=minor_manual&scope=group_project_non_inheriting&identification_complexity=supported&additional_complexity=yes&base_impact=major&pipeline_impact=major&compliance_impact=none&availability_impact=none&authorization_impact=minor&API_impact=none

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