Create ci_namespace_mirrors and ci_project_mirrors tables

What does this MR do and why?

This MR creates ci_namespace_mirrors and ci_project_mirrors tables. These tables will be used for mirroring namespace and project tables. These new models are empty for now and will be filled by !75517 (merged).

NOTE: For now, this MR is based on the branch track-deletions-in-namespaces-and-projects of !75559 (merged).

DB

UP

== 20211011140930 CreateCiNamespaceMirrors: migrating =========================
-- create_table(:ci_namespace_mirrors, {})
   -> 0.0103s
== 20211011140930 CreateCiNamespaceMirrors: migrated (0.0103s) ================

== 20211011140931 CreateCiProjectMirrors: migrating ===========================
-- create_table(:ci_project_mirrors, {})
   -> 0.0061s
== 20211011140931 CreateCiProjectMirrors: migrated (0.0062s) ==================

DOWN

== 20211011140931 CreateCiProjectMirrors: reverting ===========================
-- drop_table(:ci_project_mirrors, {})
   -> 0.0027s
== 20211011140931 CreateCiProjectMirrors: reverted (0.0043s) ==================

== 20211011140930 CreateCiNamespaceMirrors: reverting =========================
-- drop_table(:ci_namespace_mirrors, {})
   -> 0.0029s
== 20211011140930 CreateCiNamespaceMirrors: reverted (0.0044s) ================

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.