Use macOS-15 runner

Author: tmcgilchrist

.github/workflows/main.yml

@@ -1,8 +1,10 @@
 name: main
 
 on:
-  pull_request:
   push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
   schedule:
     # Prime the caches every Monday
     - cron: 0 1 * * MON
@@ -13,12 +15,12 @@ jobs:
     strategy:
       matrix:
         ocaml-compiler:
-          - 5.1.1
-          - 5.2.0
+          - 5.1
+          - 5.2
+          - 5.3
         os:
           - ubuntu-latest
-          - macos-14
-          - macos-14-large
+          - macos-15
 
     runs-on: ${{ matrix.os }}