.github(tests): bump macos-12 to macos-13 (#428)

From a GitHub blog post last month [1]:

    macOS12 runner image

    We are beginning the deprecation process for the macOS 12 runner
    image, which allows us to balance our fleet capacity ahead of our
    upcoming macOS 15 launch. This image will be fully retired by the
    December 3rd, 2024. We recommend updating workflows to use
    `macos-14`, `macos-13`, or `macos-latest`.

Note that changing to `macos-14` would be a change from x86_64 to
arm64 [2], so let's consider that separately.

[1] https://github.blog/changelog/2024-08-19-notice-of-upcoming-deprecations-and-breaking-changes-in-github-actions-runners/
[2] https://github.com/actions/runner-images/blob/370d12f1b66f/README.md#available-images

Co-authored-by: ee7 <[email protected]>

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
   test-exercises:
     strategy:
       matrix:
-        os: [ubuntu-22.04, macos-12, windows-2022]
+        os: [ubuntu-22.04, macos-13, windows-2022]
 
     name: Check exercises (${{ matrix.os }})
     runs-on: ${{ matrix.os }}