Use latest stable WP version for wp-since generation

[ernilambar]

What

The "Generate WP Functions Since Data" workflow was checking out WordPress/WordPress with no ref, so it used the default branch instead of a stable release.

Change

• Resolve the current stable version from the WordPress.org version-check API: https://api.wordpress.org/core/version-check/1.7/
• Check out the matching release tag for that version so the generated @since dataset reflects latest stable core.

[github-actions]

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Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>

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