Editor: Revert core/search block markup change.

The `src/wp-includes/blocks/*.php` render files are generated from the
Gutenberg build via tools/gutenberg/copy.js, so editing search.php
directly here is reverted by the build and fails the `git diff
--exit-code` CI check.

The `<search>` element change for the core/search block is handled in
the Gutenberg repository instead, and will flow into Core through the
next block package sync. This PR now covers only the Core-maintained
`get_search_form()` function and the bundled classic theme
`searchform.php` templates.

Author: adamsilverstein

src/wp-includes/blocks/search.php

@@ -189,7 +189,7 @@ function render_block_core_search( $attributes ) {
 	}
 
 	return sprintf(
-		'<search><form method="get" action="%1s" %2s %3s>%4s</form></search>',
+		'<form role="search" method="get" action="%1s" %2s %3s>%4s</form>',
 		esc_url( home_url( '/' ) ),
 		$wrapper_attributes,
 		$form_directives,