Upgrade PHPSpreadSheet to 1.28.0

Testing scenario 1 Login as admin Go to  http://yourmoodleurl/lib/tests/other/spreadsheettestpage.php and generate the Excel and ODS sample file. Review the server error logs to make sure there are no notices there. Open the Excel and ODS sample file in various office suites. Expected Result: Check the content is displayed as expected (Font colors, headings etc). You can download the ODS file and compare it with or download the Excel file without that patch. Alternatively, you can also compare to the moodletest_withoutpatch.ods  attached file (which was created without the patch). Testing scenario 2 Login as admin. Create a course with some enrolled students and one graded activity for some of them (at least). Access to the course. Go to Grades and use the tertiary nav to select 'Export' Click the Download button Expected Result:  Check the file is downloaded and contains one row per user and, at least, the following columns:  "First name", "Surname", "ID number", "Institution", "Department", "Email address", "Course total", "Last downloaded from this course" and one column for each activity.