MDL-57487 mod_forum: deprecate strings used in xxx_print_overview

mod/forum/lang/en/deprecated.txt

@@ -1 +1,3 @@
 inpagereplysubject,mod_forum
+overviewnumpostssince,mod_forum
+overviewnumunread,mod_forum

mod/forum/lang/en/forum.php

@@ -438,8 +438,6 @@ $string['numberofreplies'] = 'Number of replies: {$a}';
 $string['olderdiscussions'] = 'Older discussions';
 $string['oldertopics'] = 'Older topics';
 $string['oldpostdays'] = 'Read after days';
-$string['overviewnumpostssince'] = '{$a} posts since last login';
-$string['overviewnumunread'] = '{$a} total unread';
 $string['page-mod-forum-x'] = 'Any forum module page';
 $string['page-mod-forum-view'] = 'Forum module main page';
 $string['page-mod-forum-discuss'] = 'Forum module discussion thread page';
@@ -675,3 +673,5 @@ $string['forumbodydeleted'] = 'The content of this forum post has been removed a
 
 // Deprecated since Moodle 3.8.
 $string['inpagereplysubject'] = 'Re: {$a}';
+$string['overviewnumpostssince'] = '{$a} posts since last login';
+$string['overviewnumunread'] = '{$a} total unread';