Run the CI test matrix below Ruby 3.2 on ubuntu-24.04

## Motivation and Context

`ubuntu-latest` moved to ubuntu-26.04 on 2026-11-01, and `ruby/setup-ruby` has builds of CRuby 3.2 and newer only
for that image, since older CRuby versions do not compile on it. The test matrix still covers 2.7.0 (the minimum
supported version), 3.0, and 3.1, so the "Test Ruby 2.7.0" job of main a93fbf8 failed with "Unavailable version
2.7.0 for ruby on ubuntu-26.04" before running anything, and fail-fast cancelled the other Ruby jobs.

Each matrix entry now names its runner: 2.7.0, 3.0, and 3.1 run on ubuntu-24.04, where ruby-builder still provides
them, and 3.2 and newer and head keep following `ubuntu-latest`, as the RuboCop project did for its own matrix.
RuboCop, YARD, and the conformance workflow run on Ruby 4.0 and are unchanged.

## How Has This Been Tested?

The workflow file parses as YAML with the Ruby versions and steps unchanged; the pull request's own CI run shows
"Test Ruby 2.7.0" running again.

## Breaking Changes

None.

Author: koic

.github/workflows/ci.yml

@@ -3,20 +3,21 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    # ruby/setup-ruby has no CRuby below 3.2 for ubuntu-26.04, which ubuntu-latest points to, so those run on ubuntu-24.04.
+    runs-on: ${{ matrix.entry.os }}
     permissions:
       contents: read
     strategy:
       matrix:
         entry:
-          - { ruby: '2.7.0', allowed-failure: false } # Minimum supported Ruby version.
-          - { ruby: '3.0', allowed-failure: false }
-          - { ruby: '3.1', allowed-failure: false }
-          - { ruby: '3.2', allowed-failure: false }
-          - { ruby: '3.3', allowed-failure: false }
-          - { ruby: '3.4', allowed-failure: false }
-          - { ruby: '4.0', allowed-failure: false }
-          - { ruby: 'head', allowed-failure: true }
+          - { ruby: '2.7.0', os: ubuntu-24.04, allowed-failure: false } # Minimum supported Ruby version.
+          - { ruby: '3.0', os: ubuntu-24.04, allowed-failure: false }
+          - { ruby: '3.1', os: ubuntu-24.04, allowed-failure: false }
+          - { ruby: '3.2', os: ubuntu-latest, allowed-failure: false }
+          - { ruby: '3.3', os: ubuntu-latest, allowed-failure: false }
+          - { ruby: '3.4', os: ubuntu-latest, allowed-failure: false }
+          - { ruby: '4.0', os: ubuntu-latest, allowed-failure: false }
+          - { ruby: 'head', os: ubuntu-latest, allowed-failure: true }
     name: Test Ruby ${{ matrix.entry.ruby }}
     steps:
       - uses: actions/checkout@v7