MDL-80839 core_course: Recover addnew string

addnew string was deprecated in MDL-80418 but it's still in use.

lang/en/deprecated.txt

@@ -133,7 +133,6 @@ topichide,core
 summary_help,core
 editsummary,core
 format_help,core
-addnew,core
 addinganewto,core
 editinga,core
 updatinga,core

lang/en/moodle.php

@@ -64,6 +64,7 @@
 $string['addfilehere'] = 'Drop files here to add them at the bottom of this section';
 $string['addinganew'] = 'New {$a}';
 $string['additionalcustomnav'] = 'Additional custom navigation';
+$string['addnew'] = 'Add a new {$a}';
 $string['addingdatatoexisting'] = 'Adding data to existing';
 $string['additionalnames'] = 'Additional names';
 $string['addlinkhere'] = 'Add link here';
@@ -2497,7 +2498,6 @@
 * Social - The course is centred around a main forum on the course page. Additional activities and resources can be added using the Social activities block.
 * Custom sections - The course is divided into customisable sections.
 * Weekly sections - The course is divided into sections corresponding to each week, beginning from the course start date.';
-$string['addnew'] = 'Add a new {$a}';
 $string['addinganewto'] = 'Adding a new {$a->what} to {$a->to}';
 $string['editinga'] = 'Editing {$a}';
 $string['updatinga'] = 'Updating: {$a}';