Reduce the number of CI jobs since it is becoming impractical

* It was > 100 jobs and the GitHub limits to 100 jobs.

Author: eregon

.github/workflows/test.yml

@@ -15,9 +15,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, ubuntu-20.04, macos-10.15, macos-11.0, windows-2016, windows-2019 ]
+        os: [ ubuntu-18.04, ubuntu-20.04, macos-10.15, macos-11.0, windows-2016, windows-2019 ]
         # Use various version syntax here for testing
-        ruby: [ 2.1, 2.2, 2.3, 2.4, 2.5, 2.6.6, 2.7, '3.0', ruby-head, debug, jruby-9.1, jruby, jruby-head, truffleruby, truffleruby-head ]
+        ruby: [ 2.1, 2.2, 2.3, 2.4, 2.5, 2.6.6, 2.7, '3.0', ruby-head, jruby, jruby-head, truffleruby, truffleruby-head ]
         include:
         - { os: windows-2016, ruby: mingw }
         - { os: windows-2019, ruby: mingw }