chore(.github): group dependabot PRs and reduce frequency

[mafredri]

This project has zero dependencies, so dependabot PRs are mainly noise. This PR ensures all thirdparty deps are grouped together and changes the frequency from weekly to monthly.

[mtojek]

minor comment, otherwise 👍

[matifali]

Why can't we group while keeping the single config?
I guess GitHub doesn't support that.

[mafredri]

Why can't we group while keeping the single config? I guess GitHub doesn't support that.

I kept the groups separate to signify importance, where importance goes in the order of / > /internal/examples > /internal/thirdparty. We don't have deps in / though, so that's just future-proofing.

Even /internal/examples is just one dep, so that's future proofing as well.

If it's preferable we can just combine the two:

  - package-ecosystem: 'gomod'
    directories:
      - '/internal/examples'
      - '/internal/thirdparty'
    schedule:
      interval: 'monthly'
    commit-message:
      prefix: 'chore'
    labels: []
    groups:
      internal:
        patterns:
          - '*'

Sound better @matifali @mtojek?

EDIT: Combined.