Remove Ubuntu Focal from testing targets for Linux

Swift 6.2 dropped support for Ubuntu 20.04 Focal

Author: iCharlesHu

.github/workflows/pull_request.yml

@@ -9,10 +9,10 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_os_versions: '["amazonlinux2", "bookworm", "noble", "jammy", "focal", "rhel-ubi9"]'
+      linux_os_versions: '["amazonlinux2", "bookworm", "noble", "jammy", "rhel-ubi9"]'
       linux_swift_versions: '["6.1", "nightly-main"]'
       linux_pre_build_command: |
-        if command -v apt-get >/dev/null 2>&1 ; then # bookworm, noble, jammy, focal
+        if command -v apt-get >/dev/null 2>&1 ; then # bookworm, noble, jammy
           apt-get update -y
 
           # Test dependencies