With https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34480 we
introduced new jsonb column to store CI secrets configuration. Because
this is Premium feature validation was done in EE only wich caused test
failure for FOSS - https://gitlab.com/gitlab-org/gitlab/-/issues/223195.

This moves validation to app/models/ci/build_metadata.rb, we already had
to have the json schema under app/validators/json_schemas/ anyway.