Remove `day` option in dependabot scheduling

[llucax]

Using day: "[weekday name]" with monthly seems to be wrong/unsupported.

Dependabot docs say:

• Use monthly to run on the first day of each month.

We should remove the day option or use cron as interval

Originally posted by @llucax in frequenz-floss/frequenz-api-common#415 (comment)