Replace experimental @wordpress/components APIs with @wordpress/ui

[torounit]

Summary

• Replace __experimentalHStack / __experimentalHeading / __experimentalSpacer (from @wordpress/components) with @wordpress/ui's Stack and Text in the "Schedule Terms" popover header (src/editor/components/DatetimeControl.tsx).
• Stack's justify="space-between" replaces the HStack + Spacer combo (no direct Spacer equivalent exists in @wordpress/ui).
• Text is rendered as an <h2> via its render prop to preserve the original heading semantics.
• Removes the eslint-disable @wordpress/no-unsafe-wp-apis block that was previously needed for this file.
• Adds @wordpress/ui (^0.15.1) as a dependency. No extra theme/CSS setup is needed since this component renders inside the block editor, where @wordpress/ui's docs say styling is handled by the surrounding Gutenberg context.

Note: @wordpress/ui is still a 0.x package and its own README says it's "still experimental" (i.e. its API may have breaking changes across minor versions), even though it doesn't use the __experimental naming convention that the lint rule checks for.

Test plan

• npm run lint:type passes
• npm run lint-js passes (no more @wordpress/no-unsafe-wp-apis warnings)
• npm run build succeeds
• Verified visually in wp-env: opened the "Schedule Terms" panel's Attach/Detach dropdown on a post and confirmed the title and close button are laid out the same as before (title left, close button pushed to the right)