Disables Single Table Inheritance

What does this MR do and why?

Related issue: #431437 (closed)

Disables Single Table Inheritance

Allow legacy usages for now. This only affects development, and test as we change nothing for production.

This disabling can be skipped by setting SKIP_STI_CHECK to 1 (for example, if GitLab JH encounters broken pipelines because of this)

See https://docs.gitlab.com/ee/development/database/single_table_inheritance.html

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