Create single template via site editor if not existing

[luminuu]

What?

This PR changes the slug of the single template from single-post to single, which is the default slug for the single (post) template.

Why?

This PR fixes the issue #40814, where it was discovered that the template for single is not createable from the site editor when this template is not yet existing.

How?

Previously the single template was addressed as single-post, but this is not existing in the template hierarchy as default single template. The post suffix has been removed from all occurences.

Testing Instructions

1. Have an environment with WP 6.0 RC 1 available.
2. Activate a block theme with no single template available, for example Miniblock OOAK.
3. Go to the site editor.
4. Open the templates overview.
5. Click on the "Add" button and see the single template in the list of available default templates.

Screenshots or screencast

[github-actions]

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[scruffian]

Thanks for taking care of this. LGTM

[carolinan]

Creating the new template works well. But are there any other potential issues we should test for with the rename? Any compatibility issues for existing themes and plugins?

[luminuu]

Creating the new template works well. But are there any other potential issues we should test for with the rename? Any compatibility issues for existing themes and plugins?

I've searched the entire repository for single-post and except for one entry in the changelog I edited all occurences - which were only the three files in the commit. I don't think existing themes will have an issue with this, if there is a single.html template file existing, you're not going to see the entry at the Add button.