ci: update test matrix to include Ruby 3.0

.github/workflows/build.yml

@@ -12,17 +12,20 @@ jobs:
     strategy:
       matrix:
         version:
-          - 2.4
-          - 2.5
-          - 2.6
-          - 2.7
+          - "2.4"
+          - "2.5"
+          - "2.6"
+          - "2.7"
+          - "3.0"
         gemfile:
           - Gemfile_ar41
           - Gemfile_ar50
           - Gemfile_ar51
           - Gemfile_ar_master
         exclude:
-          - version: 2.7
+          - version: "2.7"
+            gemfile: Gemfile_ar41
+          - version: "3.0"
             gemfile: Gemfile_ar41
     steps:
       - uses: actions/checkout@v2