Move `saml_providers` to gitlab_main_cell

What does this MR do and why?

This MR moves saml_providers to the gitlab_main_cell schema. This is because the records in this table belongs to group_id, and namespaces are cell-local.

✅ - no cross-joins arise from this table.

1️⃣ foreign key needs to be converted to Loose foreign key because of this change and it has been marked.

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Before	After

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MR acceptance checklist

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Related to #415204 (closed)