Draft: Freeze `let_it_be` by default

What does this MR do and why?

This MRs enables freezing let_it_be by default.

Sadly, some specs fail due to this change so this MR introduces .let_it_be_frozen_exclude_todo to exclude all those files. They can be fixed later.

See !70603 (comment 696325176) for more context.

MR acceptance checklist

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