build: Update macOS runner

macos-12 will be removed on December 3.¹ Though we initially started
using macos-12 to replace macos-latest², it is now the earliest. I think
we can continue using the earliest instead of latest, for stability.

¹ <https://github.blog/changelog/2024-08-19-notice-of-upcoming-deprecations-and-breaking-changes-in-github-actions-runners/>
² "dev: Specify explicit OS image versions for Ubuntu and macOS" (3ea657e)

.github/workflows/ci.yaml

@@ -42,7 +42,7 @@ jobs:
       matrix:
         os:
           - ubuntu-22.04
-          - macos-12
+          - macos-13
 
     name: build and test (os=${{ matrix.os }})
     runs-on: ${{ matrix.os }}