Fix Prompt_Builder::using_abilities return type.

[humanik]

It should return a static type for correct typing in child classes like Prompt_Builder_With_WP_Error.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @humanik.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Unlinked contributors: humanik.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.