Security: Replace update nag to indicate end of support for 4.3.

Replaces the update nag to indicate that users need to update to a newer version of WordPress in order to continue receiving security updates.

Props audrasjb, peterwilsoncc, SirLouen

See #63595

git-svn-id: https://develop.svn.wordpress.org/branches/4.3@60335 602fd350-edb4-49c9-b593-d223f7449a82

Author: johnbillion

src/wp-admin/css/common.css

@@ -1377,6 +1377,26 @@ div.error {
 	box-shadow: 0 1px 1px 0 rgba(0,0,0,0.1);
 }
 
+.update-nag-core-insecure {
+	color: #fff;
+	background-color: #ac1b1b;
+	border-left-color: #cd5a5a;
+	font-weight: bold;
+}
+
+.update-nag-core-insecure > p:first-child {
+	margin-top: 0;
+}
+
+.update-nag-core-insecure > p:last-child {
+	margin-bottom: 0;
+}
+
+.update-nag-core-insecure a {
+	color: #fff;
+	text-decoration: underline;
+}
+
 .update-message {
 	color: #000;
 }

src/wp-admin/includes/update.php

@@ -232,10 +232,24 @@ function update_nag() {
 
 	if ( current_user_can('update_core') ) {
 		$msg = sprintf( __('<a href="https://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! <a href="%2$s">Please update now</a>.'), $cur->current, network_admin_url( 'update-core.php' ) );
+
+		$msg_line2 = sprintf(
+			/* translators: 1: WordPress version number, 2: Link to update WordPress */
+			__( 'Important! Your version of WordPress (%1$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>.' ),
+			get_bloginfo( 'version', 'display' ),
+			network_admin_url( 'update-core.php' )
+		);
 	} else {
 		$msg = sprintf( __('<a href="https://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! Please notify the site administrator.'), $cur->current );
+
+		$msg_line2 = sprintf(
+			/* translators: 1: WordPress version number, 2: Link to update WordPress */
+			__( 'Important! Your version of WordPress (%1$s) is no longer supported, you will not receive any security updates for your website. To keep your site secure, please <a href="%2$s">update to the latest version of WordPress</a>.' ),
+			get_bloginfo( 'version', 'display' ),
+			__( 'https://wordpress.org/download/' )
+		);
 	}
-	echo "<div class='update-nag'>$msg</div>";
+	echo "<div class='update-nag update-nag-core-insecure'><p>$msg</p><p>$msg_line2</p></div>";
 }
 
 // Called directly from dashboard