Popover: Use anchor instead of anchorRef in story

[tyxla]

What?

Fixes a warning in the Popover stories.

Why?

The anchor prop is recommended in favor of the deprecated anchorRef.
Found while working on #67574.

How?

We're updating the story to reflect that anchor is now recommended in favor of the deprecated anchorRef prop.

Testing Instructions

• Open the Popover > All Placements story
• Verify the anchorRef prop warning no longer appears

Testing Instructions for Keyboard

Same

Screenshots or screencast

Warning before this PR:

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Co-authored-by: tyxla <tyxla@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Thanks, @tyxla!

This matches the patterns used in the editor ✅