Dependabot config - Use "allow" option for filtering

[geriux]

Follow-up of https://github.com/wordpress-mobile/gutenberg-mobile/pull/6745

The new config worked but using "groups" for filtering duplicated the PRs for each submodule due to having two configurations:

This PR uses the allow option to specify the submodules for each config.

To test:

PR submission checklist:

• [ ] I have considered adding unit tests where possible.
• [ ] I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.