replace macos-12 with macos-13 in CI

because macos-12 will be removed soon. We still need tests on Intel
Mac.

actions/runner-images#10721

Author: rhysd

.github/workflows/ci.yaml

@@ -6,7 +6,7 @@ jobs:
     name: Unit tests
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest, macos-12]
+        os: [ubuntu-latest, macos-latest, windows-latest, macos-13]
         go: ['1.22', '1.23']
     runs-on: ${{ matrix.os }}
     steps: