GitHub Actions: Drop Ruby 2.4 tasks and Add Ruby 3.0

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

Author: cosmo0920

.github/workflows/linux.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7' ]
+        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
         postgres: [ '9', '10', '11', '12', '13' ]
         os:
           - ubuntu-latest