Remove dependabot config in favour of upstream (#1247)

Rails has started providing a `dependabot.yml` file, so instead of
maintaining our own we can rely on that instead.

See: #1243

Author: nickcharlton

lib/generators/suspenders/ci_generator.rb

@@ -6,16 +6,14 @@ class CiGenerator < Rails::Generators::Base
 
       source_root File.expand_path("../../templates/ci", __FILE__)
       desc <<~MARKDOWN
-        Uses [GitHub Actions][] for CI, and [Dependabot][] for dependency updates.
+        Uses [GitHub Actions][] for CI
 
         [GitHub Actions]: https://docs.github.com/en/actions
-        [Dependabot]: https://docs.github.com/en/code-security/dependabot/working-with-dependabot
       MARKDOWN
 
       def ci_files
         empty_directory ".github/workflows"
         template "ci.yml", ".github/workflows/ci.yml"
-        template "dependabot.yml", ".github/dependabot.yaml"
       end
 
       private

lib/generators/templates/ci/dependabot.yml

@@ -15,7 +15,6 @@ class CiGeneratorTest < Rails::Generators::TestCase
           run_generator
 
           assert_file app_root(".github/workflows/ci.yml")
-          assert_file app_root(".github/dependabot.yaml")
         end
       end
 
@@ -25,7 +24,6 @@ class CiGeneratorTest < Rails::Generators::TestCase
             run_generator
 
             assert_no_file app_root(".github/workflows/ci.yml")
-            assert_no_file app_root(".github/dependabot.yaml")
           end
         end
       end