Adjust dependabot frequency

[RobertMueller2]

Dependabot PRs may cause a bit much noise as they are configured now.

Suggestion for improved approach:

• Compatible updates (minor + patch?) of direct deps grouped once a week
• incompatible updates (major?) of direct deps standalone and once a week
• nested (indirect) deps grouped once a month

Of course depending on whether that's actually possible 😛