Change schema of multiple tables

What does this MR do and why?

While going through database dictionary files, I noticed that security_findings has gitlab_schema of gitlab_main, but it has a sharding key defined.

On digging further, I noticed a total have 4 tables have this discrepancy, ie, they have a sharding key or desired sharding key defined, but the schema is still assigned as gitlab_main, whereas it should be gitlab_main_cell instead.

I've corrected this and also added specs to make sure that this is not allowed from now.

The added specs would function like so:

Sharding key spec	Desired sharding key spec

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