Add Ruby 3.0 task

Signed-off-by: Hiroshi Hatake <cosmo0920.oucc@gmail.com>

Author: cosmo0920

.github/workflows/linux.yml

@@ -8,7 +8,7 @@ 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 }}