Fix Discord old backup remove message to include old_backup_days.

[makotsuba]

Fix Discord old backup remove message to include old_backup_days.

Context

Issue thijsvanloef#501 that I drafted was modified by thijsvanloef#502.
The message is now sent, but old_backup_days is broken.
So I fixed it.

Choices

To apply old_backup_message (e.g. 30).

Test instructions

1. I built my own image with the code changes and ran the backup command on it to see the messages sent to Discord.

Checklist before requesting a review

• I have performed a self-review/test of my code
• I've added documentation about this change to the docs.
• I've not introduced breaking changes.
• My changes do not violate linting rules.