Require approvals for Verify Backend code

What does this MR do and why?

Related to Enforce domain expert approval in critical CI a... (#393864 - closed)

In this MR we are splitting the Verify section of CODEOWNERS which was optional by default into 2 sections:

• Verify Backend that requires approval
• Verify Frontend that remains optional

In this MR we are using a newly released feature that allows to define default codeowners per section: Define default owners for CODEOWNERS sections (#371711 - closed)

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