External Libraries: First pass at fixing jQuery deprecations in WordPress core and bundled themes.

To be able to disable jQuery Migrate as step 3 of updating the jQuery version shipped with WordPress, all `JQMIGRATE` warnings in the browser console will have to be addressed.

This includes many minor adjustments to a wide array of core files.

Follow-up to:
* Step 1: Disabling jQuery Migrate 1.4.1 in WordPress 5.5: [48323], [48324]
* Step 2: Updating jQuery to 3.5.1 and adding jQuery Migrate 3.3.x in WordPress 5.6: [49101], [49338], [49615], [49649]

Props Clorith, azaozz.
See #51812.
Built from https://develop.svn.wordpress.org/trunk@50001


git-svn-id: http://core.svn.wordpress.org/trunk@49702 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/theme-plugin-editor.js

@@ -101,7 +101,7 @@ wp.themePluginEditor = (function( $ ) {
 		// Reveal the modal and set focus on the go back button.
 		component.warning
 			.removeClass( 'hidden' )
-			.find( '.file-editor-warning-go-back' ).focus();
+			.find( '.file-editor-warning-go-back' ).trigger( 'focus' );
 		// Get the links and buttons within the modal.
 		component.warningTabbables = component.warning.find( 'a, button' );
 		// Attach event handlers.