Stop testing on ubuntu-20.04 since it's gone

Author: eregon

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-20.04, ubuntu-22.04, ubuntu-24.04, ubuntu-22.04-arm, ubuntu-24.04-arm, macos-13, macos-14, macos-15, windows-2019, windows-2022, windows-2025 ]
+        os: [ ubuntu-22.04, ubuntu-24.04, ubuntu-22.04-arm, ubuntu-24.04-arm, macos-13, macos-14, macos-15, windows-2019, windows-2022, windows-2025 ]
         ruby: [
           '1.9', '2.0', '2.1', '2.2', '2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', ruby-head,
           jruby, jruby-head,