Tweak GitHub Actions

* Add Ruby 3.0 and ruby-head tasks
* Remove Ruby 2.4 task

Signed-off-by: Hiroshi Hatake <[email protected]>

Author: cosmo0920

.github/workflows/linux.yml

@@ -5,12 +5,19 @@ on:
 jobs:
   build:
     runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ matrix.experimental }}
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7' ]
+        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
         os:
           - ubuntu-latest
+        experimental: [false]
+        include:
+          - ruby: head
+            os: ubuntu-latest
+            experimental: true
+
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2