Fix aspect ratio typo and recalculate padding in embed block CSS

[gregsullivan]

Description

As discussed with @jasmussen here, there is a typo in the selector for the 9:16 aspect ratio. The ratio is listed accurately here:

gutenberg/packages/block-library/src/embed/style.scss

Lines 19 to 26 in 429558a

	// Add responsiveness to common aspect ratios.
	&.wp-embed-aspect-21-9 .wp-block-embed__wrapper,
	&.wp-embed-aspect-18-9 .wp-block-embed__wrapper,
	&.wp-embed-aspect-16-9 .wp-block-embed__wrapper,
	&.wp-embed-aspect-4-3 .wp-block-embed__wrapper,
	&.wp-embed-aspect-1-1 .wp-block-embed__wrapper,
	&.wp-embed-aspect-9-16 .wp-block-embed__wrapper,
	&.wp-embed-aspect-1-2 .wp-block-embed__wrapper {

But here it has been changed to 9:6:

gutenberg/packages/block-library/src/embed/style.scss

Lines 66 to 68 in 429558a

	&.wp-embed-aspect-9-6 .wp-block-embed__wrapper::before {
	padding-top: 66.66%; // 6 / 9 * 100
	}

Given that aspect ratios are sorted in ascending order by the required top padding, it appears the second code excerpt should be:

&.wp-embed-aspect-9-16 .wp-block-embed__wrapper::before {
	padding-top: 177.77%; // 16 / 9 * 100
}

This pull request fixes the typo and recalculates the value for padding-top.

I considered whether the intention could be to use 9:6 as an aspect ratio in addition to 9:16, but presumably that would have been listed as 3:2, so I believe 9:16 was the intended aspect ratio all along.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[youknowriad]

Thanks for the fix 🎉