Run CI with Ruby3.0

Author: okkez

.github/workflows/linux.yml

@@ -8,7 +8,12 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7' ]
+        ruby:
+          - 2.4
+          - 2.5
+          - 2.6
+          - 2.7
+          - 3.0
         os:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}