upgrade simple-captcha dependency to fix audio GC issue
This upgrade the dependency to bf7fffc (Delete leftover temporary audio files, 2023-12-19) which includes the patch identified by @stephen as fixing this issue, but also more small tweaks.
We don't update directly to latest head because there's a bunch more patches there. I've audited the changes from 0.6.0 to that commit (e7232a0..bf7fffc) and they look sane.
The .lock file was updated with an older version of poetry with:
poetry lock --no-update
... hopefully that should still work.
Closes: #151 (closed)