Remove Amazon Linux 2 from CI

Amazon Linux 2 reached end of life on 2026-06-30, and Swift.org has
stopped publishing the nightly toolchains for main and release/6.4.x.
The static Linux SDK build on the amazonlinux2 host now fails.

Remove it from CI, and also remove the now-dead yum branch from the
pre-build command, since Amazon Linux 2 was the only distribution in
the matrix that used yum.

Author: broken-circle

.github/workflows/pull_request.yml

@@ -12,7 +12,7 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_os_versions: '["amazonlinux2", "bookworm", "noble", "jammy", "rhel-ubi9"]'
+      linux_os_versions: '["bookworm", "noble", "jammy", "rhel-ubi9"]'
       linux_swift_versions: '["6.2", "nightly-main"]'
       linux_pre_build_command: |
         if command -v apt-get >/dev/null 2>&1 ; then # bookworm, noble, jammy
@@ -29,11 +29,6 @@ jobs:
 
           # Test dependencies
           dnf install -y procps
-        elif command -v yum >/dev/null 2>&1 ; then # amazonlinux2
-          yum update -y
-
-          # Test dependencies
-          yum install -y procps
         fi
       linux_build_command: "swift test && swift test -c release && swift test --disable-default-traits"
       enable_freebsd_checks: true