CI: Add macos-15 (rubyjs#327)

Fixes: rubyjs#322

Author: bnoordhuis

.github/workflows/ci.yml

@@ -14,6 +14,7 @@ jobs:
         os:
           - "macos-13"
           - "macos-14" # arm64
+          - "macos-15"
           - "ubuntu-20.04"
         ruby:
           - "truffleruby+graalvm"
@@ -52,6 +53,7 @@ jobs:
         os:
           - "macos-13"
           - "macos-14" # arm64
+          - "macos-15"
         ruby:
           - "ruby-3.1"
           - "ruby-3.2"