Add preferred version for WordPress in PR preview action

[fellyph]

What?

Closes #365

Why?

Currently, the Playground preview is loading WordPress version 6.9.4. Because the WordPress AI plugin requires at least version 7.0 to run properly, the preview environment is currently broken.

How?

I updated the GitHub Action configuration to set the WordPress version to beta. This ensures that the Playground environment loads the latest WordPress 7.0 Release Candidate (RC), making the plugin compatible again.

Use of AI Tools

Testing Instructions

1. Let the GitHub Action run on this PR.
2. Click the generated "Preview" button.
3. Verify that the Playground loads WordPress 7.0 (Beta/RC).
4. Verify that the AI plugin activates and runs successfully.

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: fellyph <fellyph@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.95%. Comparing base (5af7651) to head (1327465).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #366   +/-   ##
==========================================
  Coverage      57.95%   57.95%           
  Complexity       629      629           
==========================================
  Files             46       46           
  Lines           3204     3204           
==========================================
  Hits            1857     1857           
  Misses          1347     1347           

Flag	Coverage Δ
unit	57.95% <ø> (ø)

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[dkotter]

Can't actually test this until it's merged (as GitHub doesn't use workflows from forks). This looks correct to me so going to approve this