Editor: Ensure only the main query is modified when resolving template for new posts.

This adds a check for the main query in `_resolve_template_for_new_post()` to fix a 404 response when resolving template for new posts or pages caused by the wrong query being modified.

Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/40799 #40799: Ensure only the main query is modified when resolving template for new posts]

Follow-up to [52316].

Props petitphp, zieladam, poena, ndiego, gziolo, kebbet, Mamaduka, manfcarlo.
See #56058.
Built from https://develop.svn.wordpress.org/trunk@53593


git-svn-id: http://core.svn.wordpress.org/trunk@53181 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/block-template.php

@@ -312,6 +312,10 @@ function _block_template_render_without_post_block_context( $context ) {
  * @param WP_Query $wp_query Current WP_Query instance, passed by reference.
  */
 function _resolve_template_for_new_post( $wp_query ) {
+	if ( ! $wp_query->is_main_query() ) {
+		return;
+	}
+
 	remove_filter( 'pre_get_posts', '_resolve_template_for_new_post' );
 
 	// Pages.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53592';
+$wp_version = '6.1-alpha-53593';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.