mariadb: Change transaction isolation level to REPEATABLE-READ

[paladox]

It's the default in mariadb 11.x and WMF have been using this for over a decade.

I'm not entirely sure why we used the current value.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch paladox-patch-3

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