Bundled Themes: Twenty Twenty image caption is not aligned center when image is, on the front-end.

Fixes the image caption alignment issue by adding `text-align: center;` to `.wp-block-image .aligncenter figcaption`.

Props markusthiel, SergeyBiryukov, garethgillman.
Fixes #49320.
Built from https://develop.svn.wordpress.org/trunk@47689


git-svn-id: https://core.svn.wordpress.org/trunk@47466 1a063a9b-81f0-0310-95a4-ce76da25c4cd