Fixing CI mirrored tables mermaid chart

What does this MR do and why?

This is a fix for the Mermaid chart here

Joins are not allowed here should be between the two subgraphs.

How to set up and validate locally

graph LR
    subgraph CI["CI Tables"]
        E[other CI tables]
        F{queries with joins allowed}
        G[ci_project_mirrors]
        H[ci_namespace_mirrors]

        E---F
        F---G
        F---H
    end

    Main["Main Tables"]---L["⛔ ← Joins are not allowed → ⛔"]
    L---CI

    subgraph Main["Main Tables"]
        A[other main tables]
        B{queries with joins allowed}
        C[projects]
        D[namespaces]

        A---B
        B---C
        B---D
    end

You can try this on mermaid.live

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