Downgrade PHPUnit version

For some reason CI fails

See: https://github.com/sebastianbergmann/php-code-coverage/issues/878

https://framagit.org/framasoft/nextcloud/login-notes/-/jobs/1519469

Signed-off-by: Thomas Citharel <tcit@tcit.fr>
13 jobs for update-rewrite in 9 minutes and 33 seconds (queued for 7 seconds)
Name Stage Failure
failed
test-8.1: [master] Test
WARNING: nextcloud/apps/login_notes/junit.xml: no matching files 
WARNING: nextcloud/apps/login_notes/coverage/clover.unit.xml: no matching files
WARNING: nextcloud/apps/login_notes/html-coverage: no matching files
ERROR: No files to upload
Uploading artifacts...
WARNING: nextcloud/apps/login_notes/junit.xml: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1