add dependabot for GitHub actions and gomod

[Jason McNeil]

Recommendation to enable dependabot with a config similar to:

# To get started with Dependabot version updates, you'll need to specify which
# package ecosystems to update and where the package manifests are located.
# Please see the documentation for all configuration options:
# https://help.github.com/github/administering-a-repository/configuration-options-for-dependency-updates

version: 2
updates:
  # dependencies for GitHub Actions
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"
    commit-message:
      prefix: "build(github)"
      
  # Maintain dependencies for gomod
  - package-ecosystem: "gomod"
    directory: "/" # Location of package manifests
    schedule:
      interval: "weekly"
    commit-message:
      prefix: "build(gomod)"

Get started at https://github.com/casbin/redis-watcher/network/updates

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[Eric Luo]

Dependabot support is not in our plan. Manual PR is preferred for package update.