[Removes] Support for Ruby 2.X (#79)

* Changes rubocop target ruby version to 3.

* Removes ruby 2.6 and 2.7 from build matrix

Author: mitnal

.github/workflows/ruby.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0']
+        ruby-version: ['3.0']
 
     steps:
     - uses: actions/checkout@v2

.rubocop.yml

@@ -2,7 +2,7 @@
 # require: rubocop-performance
 
 AllCops:
-  TargetRubyVersion: 2.6
+  TargetRubyVersion: 3.0
   NewCops: enable
 
 Layout/CaseIndentation: