Bump Rubocop version

This commit bumps the Rubocop gem version to the latest one that is
supported for ruby 2.5 (system version for SLES 15 SP6), according to
the information available at:

https://docs.rubocop.org/rubocop/compatibility.html

Gemfile

@@ -4,5 +4,6 @@ gem 'yast-rake', '~> 0.2.20'
 
 group :development do
   gem 'gettext'
+  gem 'rubocop', '~> 1.28.0'
   gem 'rubocop-rspec', '~> 1.19.0'
 end

Gemfile.lock

@@ -1,31 +1,38 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    ast (2.4.0)
+    ast (2.4.2)
     gettext (3.2.9)
       locale (>= 2.0.5)
       text (>= 1.3.0)
     locale (2.1.2)
     packaging_rake_tasks (1.4.4)
       rake
-    parallel (1.12.1)
-    parser (2.5.0.0)
-      ast (~> 2.4.0)
-    powerpack (0.1.1)
-    rainbow (3.0.0)
+    parallel (1.24.0)
+    parser (3.3.0.5)
+      ast (~> 2.4.1)
+      racc
+    racc (1.7.3)
+    rainbow (3.1.1)
     rake (13.0.1)
-    rubocop (0.52.1)
+    regexp_parser (2.9.0)
+    rexml (3.2.6)
+    rubocop (1.28.2)
       parallel (~> 1.10)
-      parser (>= 2.4.0.2, < 3.0)
-      powerpack (~> 0.1)
+      parser (>= 3.1.0.0)
       rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.8, < 3.0)
+      rexml
+      rubocop-ast (>= 1.17.0, < 2.0)
       ruby-progressbar (~> 1.7)
-      unicode-display_width (~> 1.0, >= 1.0.1)
+      unicode-display_width (>= 1.4.0, < 3.0)
+    rubocop-ast (1.31.2)
+      parser (>= 3.3.0.4)
     rubocop-rspec (1.19.0)
       rubocop (>= 0.51.0)
-    ruby-progressbar (1.9.0)
+    ruby-progressbar (1.13.0)
     text (1.3.1)
-    unicode-display_width (1.3.0)
+    unicode-display_width (2.5.0)
     yast-rake (0.2.20)
       packaging_rake_tasks (>= 1.1.4)
       rake
@@ -35,6 +42,7 @@ PLATFORMS
 
 DEPENDENCIES
   gettext
+  rubocop (~> 1.28.0)
   rubocop-rspec (~> 1.19.0)
   yast-rake (~> 0.2.20)