Improve performance of design_at_version specs

What does this MR do and why?

This MR switches out some let! calls with let_it_be in design_at_version_spec.rb. Since the objects created in these are not changed, using let_it_be prevents these from being recreated in each individual spec, and instead once in the describe block. This significantly reduces the time it takes this spec to run observed locally.

References

• #229663 (closed)

MR acceptance checklist

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• The code follows the software design guidelines.
• Ensure automated tests exist following the testing pyramid. Add missing tests or create an issue documenting testing gaps.
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• You have fixed flaky tests related to this MR, or have explained why they can be ignored. Flaky tests have error Flaky test '<path/to/test>' was found in the list of files changed by this MR. but can be in jobs that pass with warnings.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #229663 (closed)