Use named export for the DEFAULT_PER_PAGE constant

What does this MR do?

We are slowly moving away from default exports and this MR is about that. Use named export for the DEFAULT_PER_PAGE constant and update its imports.

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Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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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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