Security: Introduce strings to indicate support status.

Add strings for use in future maintenance/security releases to indicate the security support status of the version of WordPress.

Two strings are introduced:

* indicating the version of WordPress is not receiving security updates, and,
* indicating the version of WordPress will shortly stop receiving security updates.

This change does not make use of the strings, the purpose is to make them available to translators prior to dropping support of selected versions of WordPress.

Props costdev, chesio, robinwpdeveloper, desrosj, rudlinkon, mukesh27, sumitbagthariya16.
Merges [54322] to the 5.1 branch.
See #56532.



git-svn-id: https://develop.svn.wordpress.org/branches/5.1@54439 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-admin/about.php

@@ -504,3 +504,9 @@
 
 /* translators: %s: Codex URL */
 __( 'For more information, see <a href="%s">the release notes</a>.' );
+
+/* translators: 1: WordPress version number, 2: Link to update WordPress */
+__( 'Important! Your version of WordPress (%1$s) is no longer supported, you will not receive any security updates for your website. To keep your site secure, please <a href="%2$s">update to the latest version of WordPress</a>.' );
+
+/* translators: 1: WordPress version number, 2: Link to update WordPress */
+__( 'Important! Your version of WordPress (%1$s) will stop receiving security updates in the near future. To keep your site secure, please <a href="%2$s">update to the latest version of WordPress</a>.' );