Merge dependabot configurations into a multi-ecosystem group

What is the problem or limitation you are having?

This project's dependabot configuration currently defines multiple "ecosystems" to be updated. As currently configured, these are run as separate updates.

We can minimize the number of updates by merging multiple updates into a single "multi-ecosystem group". This results in a single dependabot update.

Describe the solution you'd like

We should migrate the configuration of this repository to use a single "dependencies" multi-ecosystem group, rather than multiple independent groups.

The briefcase-template repository contains an example of how a multi-ecosystem configuration is configured.

Describe alternatives you've considered

Leave the configurations as single-ecosystem updates.

Additional context