Draft: Make `let_it_be(..., freeze: true)` the default

What does this MR do?

Freezes all let_it_be by default to prevent state leakage across examples.

It seems that using let_it_be_with_{reload,refind} makes the objects unfrozen again:

project = Project.last
project.frozen? # => false
project.freeze
project.frozen? # => true
project.reload
project.frozen? # => false

It's because of https://api.rubyonrails.org/classes/ActiveRecord/Core.html#method-i-frozen-3F.

Refs #299822 (comment 493168376)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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