Use theforeman-rubocop gem

Choose to inherit default.yml style because this rules have target ruby
and rails version included. Also dropped some cops from here because that
was either included in the inherit file or was not needed anymore and
fixed some fo the cops like Rails/Date and Gemspec/RequiredRubyVersion.

.github/workflows/unit_tests.yaml

@@ -14,3 +14,5 @@ jobs:
   rubocop:
     name: Rubocop
     uses: theforeman/actions/.github/workflows/rubocop.yml@v0
+    with:
+      command: bundle exec rubocop --parallel --format github

.rubocop.yml

@@ -1,15 +1,6 @@
-require:
-  - rubocop-performance
+inherit_gem:
+  theforeman-rubocop:
+    - default.yml
 
-Style/Documentation:
-  Enabled: false
-Lint/RaiseException:
-  Enabled: true
-Lint/StructNewOverride:
-  Enabled: true
-Style/HashEachMethods:
-  Enabled: true
-Style/HashTransformKeys:
-  Enabled: true
-Style/HashTransformValues:
-  Enabled: true
+AllCops:
+  TargetRubyVersion: 2.7

Gemfile

@@ -7,7 +7,4 @@ gemspec
 gem 'gettext', '>= 3.1.3', '< 4.0.0'
 gem 'rake', '~> 13.0'
 
-group :test do
-  gem 'rubocop', '~> 1.57.0'
-  gem 'rubocop-performance', '~> 1.5.2'
-end
+gem 'theforeman-rubocop', '~> 0.1.0'

hammer_cli_foreman_leapp.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |spec|
   spec.name          = 'hammer_cli_foreman_leapp'
   spec.version       = HammerCLIForemanLeapp.version.dup
   spec.license       = 'GPL-3.0'
-  spec.date          = Date.today.to_s
+  spec.date          = Time.now.utc.to_date.to_s
   spec.authors       = ['Foreman Leapp team']
   spec.email         = ['[email protected]']
   spec.homepage      = 'https://github.com/stejskalleos/hammer-cli-foreman-leapp'