Add newer Ubuntu runners to CI

Each Ubuntu LTS comes with a different version of OpenSSL, Linux Kernel,
etc.  And peopler are likely to deploy EventMachine applications to any
of these versions.  So there is some benefit to testing every Ubuntu LTS
release that is supported by GitHub Actions.

.github/workflows/workflow.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-20.04, macos-10.15, windows-2019 ]
+        os: [ ubuntu-24.04, ubuntu-22.04, ubuntu-20.04, macos-10.15, windows-2019 ]
         ruby: [ '2.2', '2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', head ]
         include:
           - { os: windows-2019, ruby: mingw }