Configure dependabot

[aminh-tran]

Fixes https://github.com/Shopify/services/issues/6240

measured doesn't have Dependabot set up. This PR adds the configuration for dependabot

I basically just copied the .github/dependabot.yml in the ticket (but set schedule interval to weekly instead of daily)

version: 2
registries:
  ruby-shopify:
    type: rubygems-server
    url: https://pkgs.shopify.io/basic/gems/ruby
    username: ${{secrets.RUBYGEMS_SERVER_PKGS_SHOPIFY_IO_USERNAME}}
    password: ${{secrets.RUBYGEMS_SERVER_PKGS_SHOPIFY_IO_PASSWORD}}
  github-com:
    type: git
    url: https://github.com
    username: ${{secrets.DEPENDENCIES_GITHUB_USER}}
    password: ${{secrets.DEPENDENCIES_GITHUB_TOKEN}}
updates:
  - package-ecosystem: bundler
    directory: "/"
    schedule:
      interval: daily      // I changed this to weekly
    open-pull-requests-limit: 100
    insecure-external-code-execution: allow
    registries: "*"

[aminh-tran]

/shipit