Use the smart_proxy_plugin.yml reusable GHA workflow

This is largely the same as test-gem, except it verifies that it at
least implements support for Ruby versions that the Smart Proxy wants.
It also implements the version selector by respecting SMART_PROXY_BRANCH
in the Gemfile.

.github/workflows/ruby.yml

@@ -19,7 +19,5 @@ jobs:
   test:
     name: Tests
     needs: rubocop
-    uses: theforeman/actions/.github/workflows/test-gem.yml@v0
-    with:
-      command: bundle exec rake test
+    uses: theforeman/actions/.github/workflows/smart_proxy_plugin.yml@v0
 ...

Gemfile

@@ -14,7 +14,7 @@ group :test do
   gem 'mocha', '~> 1'
   gem 'rack-test'
   gem 'rake', '~> 13'
-  gem 'smart_proxy', github: 'theforeman/smart-proxy', branch: 'develop'
+  gem 'smart_proxy', github: 'theforeman/smart-proxy', branch: ENV.fetch('SMART_PROXY_BRANCH', 'develop')
   gem 'test-unit', '~> 3'
   gem 'webmock', '~> 1'
 end