Fix horizontal flex layout in classic themes. (#60154)

Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

packages/edit-post/src/classic.scss

@@ -1,7 +1,8 @@
 // Provide baseline auto margin for centering blocks.
 // Specificity is kept at this level as many classic themes output
 // rules like figure { margin: 0; } which would break centering.
-.editor-styles-wrapper .wp-block {
+// These rules should also not apply to direct children of flex layout blocks.
+:where(.editor-styles-wrapper) :where(:not(.is-layout-flex, .is-layout-grid)) > .wp-block {
 	margin-left: auto;
 	margin-right: auto;
 }