Bugfix: Don't use too new Python syntax filter-output

Fix regression from ea9086ab where Python 3.6 print formatting was applied. It cannot be used (yet) since the backports support all the way back to Debian Stretch, which does not have a new enough Python version, leading to builds failing with errors like:

File "/usr/local/bin/filter-output", line 68 print(f'{bytes_written} {bytes_shown} {byte_counter}', file=sys.stderr) SyntaxError: invalid syntax

See example: https://salsa.debian.org/mariadb-team/mariadb-10.5/-/jobs/1016838