Fix CI

```
Either use a newer Ruby version or use a macOS image running on amd64, e.g., macos-13 or macos-12.
Note that GitHub changed the meaning of macos-latest from macos-12 (amd64) to macos-14 (arm64):
https://github.blog/changelog/2024-04-01-macos-14-sonoma-is-generally-available-and-the-latest-macos-runner-image/

If you are using a matrix of Ruby versions, a good solution is to run only < 2.6 on amd64, like so:
matrix:
  ruby: ['2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
  os: [ ubuntu-latest, macos-latest ]
  # CRuby < 2.6 does not support macos-arm64, so test those on amd64 instead
  exclude:
  - { os: macos-latest, ruby: '2.4' }
  - { os: macos-latest, ruby: '2.5' }
  include:
  - { os: macos-13, ruby: '2.4' }
  - { os: macos-13, ruby: '2.5' }

But of course you should consider dropping support for these long-EOL Rubies, which cannot even be built on recent macOS machines.
```

.github/workflows/main.yml

@@ -1,17 +1,20 @@
 name: CI
 
-on: [push,pull_request]
+on: [push, pull_request]
 
 jobs:
   build:
     strategy:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        ruby: [2.3, 2.4, 2.5, 2.6, 2.7, 3.0, head]
+        ruby: [2.6, 2.7, 3.0, head]
         exclude:
           - os: windows-latest
             ruby: head
+        include:
+          - os: macos-13
+            ruby: [2.3, 2.4, 2.5]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2