Revert `security_training_providers` to `gitlab_main_clusterwide` schema

Context: Afte the changes in !160207 (diffs), the table security_training_providers is now part of the gitlab_sec schema, but it does not have a sharding key defined.

Action item: Define a sharding key for this table (or exempt it from sharding key requirements, if that is the more appropriate solution using exempt_from_sharding: true)

Following discussion regarding the purpose of the security_training_providers we will revert its schema to gitlab_main_clusterwide