Enhancements:
Adapts the Video body assessment for Japanese so that it counts text length in characters instead of words and applies different length boundaries.
Other:
Bumps the minimum required Free version to 18.0.
Sets the WordPress tested up to version to 5.9.
Fixes a bug where a duplicate Video section would be shown in the Yoast sidebar in the post editor when Yoast SEO Premium is active in combination with WP 5.8.
