Compare against default category in the settings instead of hardcoded slug

[grappler]

What?

Get default category based of setting instead of hardcoded slug.

Why?

Fixes #41955

How?

Instead of using the hardcoded slug, the ID for the default category is fetched from the settings.

Testing Instructions

Test 1

1. Create a new post and publish
2. The suggestion for categories should show in the pre-publish panel

Test 2

1. Create a new category
2. Change the default category to the newly created category in the writing setting wp-admin/options-writing.php
3. Create a new post with the new category
4. Publish post and see the suggestion for categories in the pre-publish panel

[amustaque97]

Tested this locally and it works for me. Thank you for your contribution 👏🏻

I had a glimpse at the static check, not sure why it failed. Though it looks unrelated to your changes.

[Mamaduka]

Works as expected. Thanks, @grappler!